LeetCode 125 – Valid Palindrome
Check if string is a palindrome (ignore non-alnum, case).
Normalize comparison: two pointers l,r.
While l<r: skip non-alnum on both sides; compare lower(s[l]) == lower(s[r]); move inward.
If any mismatch → false; else true.
Time O(n), Space O(1).
Tags: Two pointers, Filtering.
LeetCode 167 – Two Sum II
Sorted array; find 1-indexed pair with sum = target.
Two pointers l=0, r=n-1.
If nums[l] + nums[r] < target → l++; if > target → r–; else return.
Invariant: sorted lets us drop a side each step.
Time O(n), Space O(1).
Tags: Two pointers, Monotone shrink.
LeetCode 15 – 3Sum
All unique triplets with sum 0.
Sort. For each i (skip duplicates & early break if nums[i]>0):
Two-sum on subarray [i+1..] with l/r.
Move l/r based on sum; after a hit, skip duplicates on both sides.
Time O(n²), Space O(1) extra.
Tags: Sorting + two pointers, Deduplication.
Pitfalls: duplicates, integer ranges.
LeetCode 11 – Container With Most Water
Max area formed by two lines.
Two pointers at ends; area = min(h[l], h[r]) * (r-l).
Move the shorter side inward (only way to possibly increase min height).
Track best.
Time O(n), Space O(1).
Tags: Greedy two pointers, Invariant on min height.
LeetCode 42 – Trapping Rain Water
Total trapped water between bars.
Two pointers with running maxima: leftMax, rightMax.
While l<r:
If leftMax ≤ rightMax: water += max(0, leftMax - height[l]); update leftMax; l++.
Else: water += max(0, rightMax - height[r]); update rightMax; r–.
Reason: lower side limits water level.
Time O(n), Space O(1).
Tags: Two pointers, Prefix/Suffix maxima (implicit).
Alt: monotonic stack (O(n), more memory).
