Explain Trapping Rain Water at a high level.

Short answer: Water at i is min(leftMax, rightMax) − height[i]. Compute with two arrays, or optimize with two pointers moving from ends while tracking leftMax/rightMax. Stack-based solution processes bars as histogram valleys.

Complexity

Two pointers O(n) time, O(1) space.

Relate it to Container With Most Water but stress per-index water units.