class Solution {
public:
    int trap(vector<int>& height) {
        int left = 0, right = height.size() - 1, ans = 0;

        while(left < right) {
            int minH = min(height[left], height[right]);

            //> start from minH to find how much water can be trapped
            if(height[left] == minH) {
                ++left;
                while(left < right && height[left] < minH) {
                    ans += minH - height[left];
                    ++left;
                }
            }
            else {
                --right;
                while(left < right && height[right] < minH) {
                    ans += minH - height[right];
                    --right;
                }
            }
        }

        return ans;
    }
};