Array

LeetCode 39 note: numbers may be reused, so the next level starts at i (not i + 1), which also avoids duplicates differing only in order

LeetCode 36 note: each digit may appear once per row, column and 3 × 3 box; put (digit, row), (digit, column) and (digit, box) keys into one set

LeetCode 34 note: two lower bounds: the first position >= target, and the first position >= target + 1 minus one

LeetCode 33 note: after rotation one side of mid is always sorted; check whether the target lies in it and search there, otherwise search the other side

LeetCode 31 note: from the right find the first dip i, swap it with the smallest larger value to its right, then reverse everything after i, with comparisons that include equality

LeetCode 18 note: sort, fix the first two numbers with two loops, and converge pointers for the rest, skipping equal values at all four positions

LeetCode 16 note: sort, fix one number, and find the other two with converging pointers, comparing the distance abs(sum - target)

LeetCode 15 note: sort, fix the first number, and find the other two with converging pointers, skipping equal values at all three positions to avoid duplicate triplets

LeetCode 11 note: start pointers at both ends and always move the shorter side; moving the taller side only narrows the width without raising the height

LeetCode 1184 note: a circular route has two directions; clockwise is the sum between start and destination, counterclockwise is the total minus that

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