Two-Pointers

LeetCode 148 note: split in half with fast and slow pointers, sort each half recursively and merge; O(n log n) with no extra array

LeetCode 142 note: after fast and slow meet, send one back to head and step both one at a time; they meet again at the cycle entry

LeetCode 151 note: split() with no argument treats runs of spaces as one separator and drops leading and trailing spaces; reverse and join with single spaces

LeetCode 189 note: take k %= n, then reverse three times: the whole array, the first k, and the last n - k, with O(1) extra space

LeetCode 647 note: expand around centers, counting one palindromic substring for each successful expansion

LeetCode 287 note: treat i -> nums[i] as a linked list; the duplicate is the cycle entry, found with Floyd’s algorithm in O(1) space without modifying the array

LeetCode 143 note: find the middle, reverse the second half, and merge the two alternately, cutting the first half’s tail

LeetCode 42 note: each cell holds min(tallest on the left, tallest on the right) - its own height; use prefix and suffix maxima or two pointers

LeetCode 167 note: sorted, so move two pointers inward, shifting right when the sum is too large and left when too small; indices are 1-based

LeetCode 86 note: two dummies collect nodes below x and at least x, then join them, setting the second part’s tail to None

LeetCode 82 note: every duplicated value goes, the head included, so use a dummy; on a duplicate skip to the next different value and link prev.next past it

LeetCode 80 note: keep each value at most twice by comparing with nums[k - 2], two slots back from the write position