String

LeetCode 763 note: record each letter’s last index, extend the current part’s end while scanning, and cut when the scan reaches it

LeetCode 32 note: a stack of indices whose bottom is the position just before the current valid run; the length is i - stack[-1]

LeetCode 4097 note: only merge the fixed pairs (0,1), (2,3), …, stepping by 2, not every adjacent pair

LeetCode 449 note: for a BST, pre-order without null markers is enough; deserialize recursively with bounds in O(n)

LeetCode 331 note: count open slots: start with 1, every node fills one, a non-null node opens two more; never go negative and end at exactly 0

LeetCode 297 note: pre-order with null markers identifies the tree uniquely; deserialize by consuming tokens from an iterator

Simulation by building a list, parsing by trying every pattern at each position, all substrings of length k, counting by character class, checking balanced substrings from a fixed start, and splitting into words

Converging pointers, one pointer per sequence, skipping blanks to compare positions, counting pairs after sorting, and the in-place write pointer

Building a Trie from nested dicts, counting on nodes, pairing it with DFS, and when a Trie is not actually needed

LeetCode 1842 note: a palindrome is determined by its left half and the middle digit stays put; take the next permutation of the left half and mirror it

Prefix tables, edit operations, subsequences, and interval palindrome DP

LeetCode 4019 note: kept characters never change again, so record each character’s last position in the result; a new character within k of it gets merged away