Counting

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 1814 note: rearrange the condition into nums[i] - rev(nums[i]) == nums[j] - rev(nums[j]), count pairs per value, and remember the modulus

LeetCode 3071 note: count each value on and off the Y, enumerate the Y value a and background value b (a != b), and take the fewest cells to change

LeetCode 229 note: at most two values can exceed n / 3; Boyer-Moore keeps two candidates and a second pass verifies their counts

LeetCode 1189 note: each letter supports count // needed balloons, with l and o needed twice; take the minimum

LeetCode 1160 note: a word is formable when none of its character counts exceeds chars’; sum the lengths of such words

LeetCode 1128 note: normalize each domino to (small, large) as a key and count; a key with c dominoes yields c(c - 1) / 2 pairs

LeetCode 914 note: every card count must be divisible by one X >= 2, i.e. the GCD of all counts is at least 2

LeetCode 884 note: count words across both sentences together; those appearing exactly once are the answer

LeetCode 819 note: replace punctuation with spaces, lowercase, split, count, and pick the most frequent word not banned

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