Two-Pointers

LeetCode 75 note: the Dutch flag, with 0s left of low and 2s right of high; after swapping a 2 to high, do not advance, since the swapped-in value is unchecked

LeetCode 61 note: count the length and close a ring, take k %= n, walk n - k - 1 steps from the head to the new tail, and break there

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 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 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 1089 note: count the zeros to know each value’s shift, then move from back to front, writing the extra 0 when copying a zero

LeetCode 977 note: the largest squares are at the two ends, so compare the ends with two pointers and fill the result from the back

LeetCode 942 note: two pointers hold the smallest and largest unused values; place the smallest on I and the largest on D