Two Sum
Hash map — Find two numbers that add to target.
3Sum
Two pointers — Find triplets summing to zero.
Maximum Subarray
Kadane's algorithm — Find contiguous max sum.
Merge Intervals
Sorting — Merge overlapping intervals.
Product of Array Except Self
Prefix/suffix — Product without division.
Rotate Array
Reversal — Rotate array k steps.
Container With Most Water
Two pointers — Max area between lines.
Trapping Rain Water
Two pointers — Water trapped between bars.