Hash-Table

LeetCode 1410 note: scan left to right, trying all six entities at each position and skipping past a match; chained replace does not work

LeetCode 3606 note: filter valid coupons, then sort by (category order, code); character ranges must include z, Z and 9

Fixed-size windows, variable windows for longest / shortest, counting subarrays (at most K minus at most K - 1, count += left for at least K), and monotonic deques for window extremes

LeetCode 2094 note: enumerate the even numbers 100 to 998 instead and check that each digit needed is available; Counter subtraction keeps only positives

LeetCode 3545 note: reducing the number of kinds means deleting whole kinds, starting from the least frequent

LeetCode 952 note: sharing a factor ⇔ sharing a prime factor; factor each number and union indices with the same prime factor using Union-Find, with no pairwise GCDs

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 3487 note: free deletion means picking distinct values; take every non-negative one, or only the maximum when all are negative

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