Trapping Rain Water - Two Pointer Approach - Two Pointers | Fundamental Algorithms

Water at position $i$ equals $\min(maxLeft, maxRight) - height[i]$ . Instead of precomputing arrays, use two pointers with running maximums.

$1.$ Track leftMax and rightMax as you go.

$2.$ If leftMax < rightMax, process left pointer. Water is bounded by leftMax.

$3.$ Otherwise process right pointer. Water is bounded by rightMax.

$4.$ The smaller of the two maxes determines the water level at that position.