https://h30306.github.io/blog/tags/breadth-first-search/Recent content in Breadth-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-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-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-429/Sat, 24 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-429/LeetCode 429 note: BFS level order with the children list in place of left and righthttps://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-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-3787/Thu, 22 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-3787/LeetCode 3787 note: two BFS passes find one diameter’s ends A and B; every node’s farthest node is A or B, so a node is a diameter endpoint exactly when max(dist(A,i), dist(B,i)) equals the diameterhttps://h30306.github.io/blog/leetcode/leetcode-773/Mon, 07 Sep 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-773/LeetCode 773 note: there are only 6! = 720 boards, so flatten each board into a string as a BFS state, with swaps of 0 and a neighbor as the moveshttps://h30306.github.io/blog/leetcode/leetcode-4041/Sun, 30 Aug 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-4041/LeetCode 4041 note: with operations in any order, BFS finds every value a number can become (up to sum) with the fewest steps, then the same grouped 0/1 knapsack as 4040https://h30306.github.io/blog/leetcode/leetcode-488/Wed, 08 Jul 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-488/LeetCode 488 note: with at most 16 balls on the board and 5 in hand, memoize DFS on the whole (board, hand) state, inserting only before a same-colored ball or between two equal ballshttps://h30306.github.io/blog/leetcode/leetcode-103/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-103/LeetCode 103 note: a normal level-order traversal with every odd level reversed gives the zigzaghttps://h30306.github.io/blog/leetcode/leetcode-107/Fri, 18 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-107/LeetCode 107 note: a normal level-order traversal reversed at the end gives bottom-up orderhttps://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-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-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-102/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-102/LeetCode 102 note: BFS one whole level at a time, building the next level from the current one; an empty tree returns an empty listhttps://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-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-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-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-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-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-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-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 tree