Counting

Counting and frequency of frequencies, using the array itself as a hash table (sign marking), counting several keys at once (rows, columns, diagonals), incremental counts from a fixed left end, and grouping by a normalized key

Merge sort templates, where to count in inversion-style problems, the linked list version, and common mistakes

LeetCode 493 note: when the counting condition differs from the sort order, scan first, then merge

LeetCode 2426 note: simplify the inequality, then decide whether the matches form a prefix or a suffix

LCR 170 note: count inversions during the merge step

LeetCode 315 note: merge sort with indices, plus a BIT solution

LeetCode 2416 note: each Trie node counts the words passing through it, and a word’s answer is the sum of counts along its path

LeetCode note for Number of Dice Rolls With Target Sum, rebuilt from the original learning note

LeetCode 3713 note: fix the start and extend right; “every character has the same count” means “highest count × kinds = length”, an O(1) check per step

LeetCode 3960 note: n is only 1000, so fix the left end and extend right while maintaining value -> count and count -> how many; exactly two counts f and 2f means balanced

LeetCode 3913 note: extract only the vowels, sort by (-count, first occurrence), and put them back into the vowel positions

LeetCode 3843 note: count each value, then count how many values share each count; the first element from the left whose count is its own is the answer