String

LeetCode 3541 note: count characters, take the maximum among vowels and among consonants and add them; a missing class counts as 0

LeetCode 3518 note: a palindrome is determined by its left half, so this is the k-th permutation of a multiset; try characters position by position and use permutation counts, updated in O(1), to find k’s branch

LeetCode 1768 note: take characters in turn with one index, then append whatever remains of the longer string

LeetCode 3136 note: length at least 3, alphanumeric only, at least one vowel and at least one consonant; digits are not consonants

LeetCode 2000 note: find the first index i of ch, reverse word[:i + 1] and append the rest; return the word unchanged if ch is absent

LeetCode 3121 note: track each letter’s last lowercase index and first uppercase index; it counts only when the former comes first

LeetCode 3120 note: collect the letters seen in lowercase and in uppercase; the size of their intersection is the answer

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

LeetCode 165 note: split on dots, compare revision by revision as integers, and treat missing revisions of the shorter version as 0

LeetCode 402 note: a monotonically increasing stack; while deletions remain, pop any top larger than the current digit, then strip leading zeros

LeetCode 187 note: count every length-10 substring in a hash map and return those seen more than once

LeetCode 227 note: read whole numbers, and at each operator apply the previous one: + and - push the number (negated for -), * and / combine with the top