LeetCode

LeetCode 43 note: num1[i] × num2[j] lands at position i + j + 1; accumulate into an array of length n + m and carry at the end

LeetCode 40 note: each number is used once and duplicates exist, so sort, start the next level at i + 1, and skip equal values within a level

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 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