Backtracking

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

LeetCode 212 note: build a Trie of all words, walk it during the grid DFS, remove words once found, and prune empty nodes

LeetCode 301 note: compute the minimum left and right deletions before backtracking, or BFS one deletion per level and stop at the first valid level

LeetCode 996 note: with n at most 12, use bitmask DP: dp[mask][last] = permutations using the set mask and ending with last; dedupe by using equal values in index order

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

LeetCode 51 note: one queen per row, with three sets tracking occupied columns and both diagonals

LeetCode 3799 note: four nested levels pruned by corner letters; sorting the words first makes the output lexicographic

Generating all valid parentheses combinations using backtracking approach

LeetCode 2044 note: the maximum OR is the OR of everything; once backtracking reaches it, count the rest as 2^k

LeetCode 988 note: DFS prepending each character so the string at a leaf already reads leaf to root, and keep the smallest

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 pruning

LeetCode 473 note: the total must be divisible by 4; place sticks from longest to shortest into one of four sides, skipping a side that would overflow or that equals a side already tried