Dfs

DFS return values, BFS level order, BST bounds, climbing to the LCA, tree diameter, and leaf trimming

Connected components, grid flood fill, multi-source BFS, state-space BFS, shortest cycles, and recursion depth in DFS

The choose, recurse, undo template for subsets, permutations, constrained placement, grid search, and pruning

LeetCode 827 note: label every island and record its size, then for each 0 add the sizes of the distinct neighboring islands plus 1

LeetCode 3203 note: connect the two centers; the answer is max(d1, d2, ceil(d1/2) + ceil(d2/2) + 1)

LeetCode 1028 note: a stack holds the path from the root to the current node, so its length is the depth

LeetCode 323 note: count components with DFS or Union-Find; Union-Find without path compression is much slower

LeetCode 1377 note: undirected edges, seen must hold nodes, and an early arrival depends on whether the frog can still move

LeetCode 980 note: grid backtracking that marks visited cells in place and checks at the end that every empty cell was covered

LeetCode 865 note: BFS to the last level and climb together, or one DFS returning (depth, node)

LeetCode 1339 note: compute the total, enumerate every subtree sum, and leave the input untouched

LeetCode 1306 note: indices are nodes and the two jumps are edges; BFS or DFS decides whether a zero is reachable