https://h30306.github.io/blog/tags/depth-first-search/Recent content in Depth-First-Search onHugo -- gohugo.ioenhwork0511@gmail.com (Wei Yi Chung)hwork0511@gmail.com (Wei Yi Chung)© 2026 Wei Yi ChungSun, 01 Nov 2026 00:00:00 +0000https://h30306.github.io/blog/leetcode/leetcode-2385/Sun, 01 Nov 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-2385/LeetCode 2385 note: turn the tree into an undirected graph and BFS from start; the number of levels minus one is the answerhttps://h30306.github.io/blog/leetcode/leetcode-2858/Sat, 31 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-2858/LeetCode 2858 note: rerooting DP: count the reversals needed with root 0, then moving the root to a child adds 1 if that edge pointed down and subtracts 1 if it pointed uphttps://h30306.github.io/blog/leetcode/leetcode-834/Sat, 31 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-834/LeetCode 834 note: rerooting DP: compute subtree sizes and ans[0] with root 0, then push ans[child] = ans[parent] - size + (n - size) to every nodehttps://h30306.github.io/blog/leetcode/leetcode-968/Fri, 30 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-968/LeetCode 968 note: post-order greedy with three states per node: has a camera, covered, not covered; place a camera only when a child is uncovered, and add one if the root is left uncoveredhttps://h30306.github.io/blog/leetcode/leetcode-437/Wed, 28 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-437/LeetCode 437 note: prefix sums on a tree with a hash map: look up prefix - target before adding the current prefix, and undo it when leaving the nodehttps://h30306.github.io/blog/leetcode/leetcode-863/Wed, 28 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-863/LeetCode 863 note: turn the tree into an undirected graph (adding edges to parents), then BFS k steps from targethttps://h30306.github.io/blog/leetcode/leetcode-652/Mon, 26 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-652/LeetCode 652 note: encode every subtree in post-order and report it on its second occurrence; string keys are O(n^2) worst case, (val, left id, right id) ids make it O(n)https://h30306.github.io/blog/leetcode/leetcode-297/Sun, 25 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-297/LeetCode 297 note: pre-order with null markers identifies the tree uniquely; deserialize by consuming tokens from an iteratorhttps://h30306.github.io/blog/leetcode/leetcode-449/Sun, 25 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-449/LeetCode 449 note: for a BST, pre-order without null markers is enough; deserialize recursively with bounds in O(n)https://h30306.github.io/blog/leetcode/leetcode-99/Sun, 25 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-99/LeetCode 99 note: find the inversions in the in-order sequence: the earlier node of the first inversion and the later node of the last one were swappedhttps://h30306.github.io/blog/leetcode/leetcode-124/Sat, 24 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-124/LeetCode 124 note: DFS returns the best downward chain and updates the answer with left chain + node + right chain at every node; drop negative chainshttps://h30306.github.io/blog/leetcode/leetcode-1372/Sat, 24 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1372/LeetCode 1372 note: each node returns two values, the longest zigzag starting left and starting right; going left extends the left child’s start-right valuehttps://h30306.github.io/blog/leetcode/leetcode-515/Sat, 24 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-515/LeetCode 515 note: BFS level order, taking the maximum of each level; start from -infhttps://h30306.github.io/blog/leetcode/leetcode-687/Sat, 24 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-687/LeetCode 687 note: DFS returns the longest same-value downward chain in edges, and each node updates the answer with left + righthttps://h30306.github.io/blog/leetcode/leetcode-314/Thu, 22 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-314/LeetCode 314 note: BFS with a column index; BFS is what guarantees top-to-bottom within a column and left-to-right within a rowhttps://h30306.github.io/blog/leetcode/leetcode-2973/Sat, 12 Sep 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-2973/LeetCode 2973 note: the max product of three is either the three largest or the two smallest times the largest, so each subtree returns only its 2 smallest and 3 largest values, merged in post-orderhttps://h30306.github.io/blog/leetcode/leetcode-1373/Thu, 10 Sep 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1373/LeetCode 1373 note: a post-order traversal returns (sum, min, max, is_bst); a node roots a BST when the left max < its value < the right minhttps://h30306.github.io/blog/leetcode/leetcode-988/Wed, 17 Apr 2024 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-988/LeetCode 988 note: DFS prepending each character so the string at a leaf already reads leaf to root, and keep the smallesthttps://h30306.github.io/blog/leetcode/leetcode-113/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-113/LeetCode 113 note: backtrack with the current path and store a copy at a leaf whose total matches; values can be negative, so no early pruninghttps://h30306.github.io/blog/leetcode/leetcode-116/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-116/LeetCode 116 note: in a perfect binary tree, walk each level through the next pointers already set on the level above: left.next = right and right.next = next.left, with O(1) extra spacehttps://h30306.github.io/blog/leetcode/leetcode-117/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-117/LeetCode 117 note: connect next pointers level by level with BFS; for a non-perfect tree, looking only at the neighbour’s children breaks the chainhttps://h30306.github.io/blog/leetcode/leetcode-129/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-129/LeetCode 129 note: on the way down multiply the current number by 10 and add the node’s value, adding it to the total at a leafhttps://h30306.github.io/blog/leetcode/leetcode-2331/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-2331/LeetCode 2331 note: leaves are 0 or 1 and internal nodes are 2 for OR and 3 for AND, evaluated in post-orderhttps://h30306.github.io/blog/leetcode/leetcode-236/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-236/LeetCode 236 note: recursion returns whichever of p or q a subtree contains; finding something on both sides means they split here, so this node is the LCAhttps://h30306.github.io/blog/leetcode/leetcode-1379/Thu, 17 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1379/LeetCode 1379 note: walk both trees in lockstep; when the original reaches target, the clone’s node at the same position is the answer, comparing node identityhttps://h30306.github.io/blog/leetcode/leetcode-337/Wed, 16 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-337/LeetCode 337 note: tree DP where each node returns the best (rob it, skip it) pair; not robbing alternate levelshttps://h30306.github.io/blog/leetcode/leetcode-133/Sun, 30 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-133/LeetCode 133 note: during DFS map each old node to its copy and return the existing copy when revisited, avoiding infinite recursion; an empty graph returns Nonehttps://h30306.github.io/blog/leetcode/leetcode-417/Sun, 30 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-417/LeetCode 417 note: walk inward from each ocean’s shore to cells no lower than the current one; cells reached from both oceans are the answerhttps://h30306.github.io/blog/leetcode/leetcode-695/Sun, 30 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-695/LeetCode 695 note: DFS each unvisited 1 to measure its island and keep the maximum; cells are the integer 1, not a stringhttps://h30306.github.io/blog/leetcode/leetcode-1448/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1448/LeetCode 1448 note: pass the path maximum down during DFS; a node is good when its value is at least that maximumhttps://h30306.github.io/blog/leetcode/leetcode-199/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-199/LeetCode 199 note: in a level-order traversal, each level’s rightmost node is what you see from the righthttps://h30306.github.io/blog/leetcode/leetcode-230/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-230/LeetCode 230 note: a BST’s in-order traversal is ascending, so stop an iterative in-order walk at the k-th nodehttps://h30306.github.io/blog/leetcode/leetcode-98/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-98/LeetCode 98 note: every node has a valid range (low, high); going left the upper bound becomes the node and going right the lower bound does; comparing parent and child is not enoughhttps://h30306.github.io/blog/leetcode/leetcode-79/Wed, 26 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-79/LeetCode 79 note: backtrack from every cell, matching the next character in four directions with visited cells marked temporarily and restored; prune early with character countshttps://h30306.github.io/blog/leetcode/leetcode-1022/Tue, 27 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1022/LeetCode 1022 note: on the way down double the current number and add the node’s bit, adding it to the total at a leafhttps://h30306.github.io/blog/leetcode/leetcode-938/Mon, 26 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-938/LeetCode 938 note: prune with the BST order, going only right below low and only left above high, adding nodes inside the rangehttps://h30306.github.io/blog/leetcode/leetcode-965/Mon, 26 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-965/LeetCode 965 note: every value must equal the root’s; compare values while recursing, not the booleans the recursion returnshttps://h30306.github.io/blog/leetcode/leetcode-993/Mon, 26 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-993/LeetCode 993 note: cousins share a depth but not a parent; record x’s and y’s depth and parent during DFS and comparehttps://h30306.github.io/blog/leetcode/leetcode-897/Sun, 25 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-897/LeetCode 897 note: in-order traversal appending nodes to a right-only chain with left children cleared; keep the dummy separate from the moving tailhttps://h30306.github.io/blog/leetcode/leetcode-872/Sat, 17 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-872/LeetCode 872 note: collect leaves left to right with DFS; BFS collects level by level, which is not left-to-right orderhttps://h30306.github.io/blog/leetcode/leetcode-783/Tue, 06 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-783/LeetCode 783 note: a BST’s in-order is ascending, so the minimum difference is between in-order neighbors; remember the previous value and comparehttps://h30306.github.io/blog/leetcode/leetcode-733/Sat, 03 Sep 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-733/LeetCode 733 note: DFS or BFS from the start, recoloring 4-directional cells that share the start’s original color; return immediately if the colors are equalhttps://h30306.github.io/blog/leetcode/leetcode-637/Mon, 29 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-637/LeetCode 637 note: level-order traversal, dividing each level’s sum by its counthttps://h30306.github.io/blog/leetcode/leetcode-653/Mon, 29 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-653/LeetCode 653 note: traverse while keeping a set of seen values; finding k - val in it means a pair existshttps://h30306.github.io/blog/leetcode/leetcode-671/Mon, 29 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-671/LeetCode 671 note: the root is the minimum; the second minimum is the smallest value larger than the root, and only subtrees equal to the root need searchinghttps://h30306.github.io/blog/leetcode/leetcode-606/Sun, 28 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-606/LeetCode 606 note: output in pre-order, keeping an empty () when only the left subtree is missing, and omitting the right parentheses when it is emptyhttps://h30306.github.io/blog/leetcode/leetcode-559/Sat, 27 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-559/LeetCode 559 note: depth is 1 plus the deepest child, or 1 with no childrenhttps://h30306.github.io/blog/leetcode/leetcode-563/Sat, 27 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-563/LeetCode 563 note: return subtree sums in post-order; each node’s tilt is the difference of its subtree sums, added to the answerhttps://h30306.github.io/blog/leetcode/leetcode-572/Sat, 27 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-572/LeetCode 572 note: for every node of root, check whether its subtree equals subRoot; recurse with isSubtree, not by comparing only the childrenhttps://h30306.github.io/blog/leetcode/leetcode-589/Sat, 27 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-589/LeetCode 589 note: n-ary pre-order records the node then each child in order; flatten recursive results with extend, not by appending a listhttps://h30306.github.io/blog/leetcode/leetcode-590/Sat, 27 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-590/LeetCode 590 note: post-order is each child then the node; iteratively, run a root-then-children-right-to-left pre-order and reverse the whole resulthttps://h30306.github.io/blog/leetcode/leetcode-501/Thu, 25 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-501/LeetCode 501 note: a BST’s in-order is sorted, so equal values are contiguous; count the current run during in-order, resetting the answer on a new maximum and appending on a tiehttps://h30306.github.io/blog/leetcode/leetcode-530/Thu, 25 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-530/LeetCode 530 note: a BST’s in-order is ascending, so the minimum difference is between in-order neighbors; remember the previous value and comparehttps://h30306.github.io/blog/leetcode/leetcode-543/Thu, 25 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-543/LeetCode 543 note: the diameter may skip the root; return heights in post-order while updating the answer with left + right height at every node, counting edgeshttps://h30306.github.io/blog/leetcode/leetcode-463/Wed, 24 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-463/LeetCode 463 note: each land cell contributes 4 edges, minus 2 for each land neighbor above or to the lefthttps://h30306.github.io/blog/leetcode/leetcode-404/Tue, 23 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-404/LeetCode 404 note: tell each child during DFS whether it is a left child, adding only left children that are leaveshttps://h30306.github.io/blog/leetcode/leetcode-235/Mon, 22 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-235/LeetCode 235 note: go left when both p and q are smaller, right when both are larger; otherwise (split, or one is the node itself) this node is the LCAhttps://h30306.github.io/blog/leetcode/leetcode-257/Mon, 22 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-257/LeetCode 257 note: DFS carrying the path string down, adding the whole path at each leafhttps://h30306.github.io/blog/leetcode/leetcode-226/Sun, 21 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-226/LeetCode 226 note: swap every node’s children, using a simultaneous assignment so the already-changed side is not reusedhttps://h30306.github.io/blog/leetcode/leetcode-145/Sat, 20 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-145/LeetCode 145 note: iterative post-order pushes a node back with a visited flag on its first pop, then the right and left children; record it on the second pophttps://h30306.github.io/blog/leetcode/leetcode-110/Tue, 16 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-110/LeetCode 110 note: every node’s subtree heights must differ by at most 1; return heights in post-order and propagate -1 once unbalancedhttps://h30306.github.io/blog/leetcode/leetcode-111/Tue, 16 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-111/LeetCode 111 note: minimum depth must end at a leaf; with only one child, the empty side (depth 0) cannot count as the minimumhttps://h30306.github.io/blog/leetcode/leetcode-112/Tue, 16 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-112/LeetCode 112 note: subtract each value from targetSum on the way down; it works if the remainder matches at a leafhttps://h30306.github.io/blog/leetcode/leetcode-100/Tue, 02 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-100/LeetCode 100 note: two empty trees are equal, one empty and one not differ, and otherwise compare values and recurse on both sideshttps://h30306.github.io/blog/leetcode/leetcode-101/Tue, 02 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-101/LeetCode 101 note: the left and right subtrees must mirror each other, outer with outer and inner with inner; compare values, not node objectshttps://h30306.github.io/blog/leetcode/leetcode-104/Tue, 02 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-104/LeetCode 104 note: depth is 1 plus the larger subtree depth, with 0 for an empty treehttps://h30306.github.io/blog/leetcode/leetcode-144/Wed, 20 Jul 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-144/LeetCode 144 note: iterative pre-order traversal records on pop and pushes the right child before the left so the left is handled firsthttps://h30306.github.io/blog/leetcode/leetcode-94/Wed, 20 Jul 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-94/LeetCode 94 note: iterative in-order traversal, pushing nodes while going left, recording on pop, then turning to the right subtree