Tree

LeetCode 1161 note: pushing None into the next level creates a fake level with sum 0

LeetCode 2236 note: a warm-up, plus practice with C++ pointer access

LeetCode 958 note: completeness is a global property, so check with BFS whether any node follows the first gap

LeetCode 1008 note: splitting the array is O(n^2); recursing with an upper bound is O(n)

LeetCode Problem Solving

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

LeetCode 109 note: take the middle found with fast and slow pointers as the root and recurse on both halves; the height balances naturally

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 LCA

LeetCode 2331 note: leaves are 0 or 1 and internal nodes are 2 for OR and 3 for AND, evaluated in post-order

LeetCode 222 note: in a complete tree, equal leftmost depths on both sides mean one side is perfect, counted as 2^h - 1, for O(log² n)

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 leaf

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 chain