Medium

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 24 note: a dummy plus pairs, rewiring prev.next, a.next and b.next together; swap nodes, not values

LeetCode 5 note: expand around centers, using each position as an odd and an even center; with interval DP, iterate i from large to small

LeetCode 19 note: from a dummy, move fast n + 1 steps ahead, then move both to the end; slow stops before the node to delete

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 17 note: each level handles one digit and tries its letters; an empty input returns an empty list

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