String

LeetCode 389 note: t has one extra character; XOR every character of both strings, pairs cancel, and the extra one remains

LeetCode 387 note: count first, then scan left to right for the first character with count 1

LeetCode 383 note: every character’s demand in ransomNote must not exceed its supply in magazine; an empty Counter difference means yes

LeetCode 345 note: the same two pointers as LC 917, with the condition switched to “is a vowel” (both cases)

LeetCode 344 note: swap with two pointers and move inward to reverse in place

LeetCode 290 note: pattern characters and words must correspond one to one; use two dicts or compare set sizes

LeetCode 257 note: DFS carrying the path string down, adding the whole path at each leaf

LeetCode 242 note: two strings are anagrams exactly when their character counts match; compare Counters or sorted strings

LeetCode 205 note: characters must map one to one; it holds when the distinct characters and distinct pairs all have the same count

LeetCode 171 note: scan left to right with res = res × 26 + letter value (A = 1)

LeetCode 168 note: Excel columns are base 26 without a zero digit; take (n - 1) % 26 as the letter, then n = (n - 1) // 26

LeetCode 125 note: two pointers skip non-alphanumeric characters and compare lowercase letters, with digits included