leetcode_42.cpp

class Solution {
    public:
        int trap(vector<int>& height) {
            if (height.empty())
                return 0;
    
            int left = 0, right = height.size() - 1;
            int left_max = 0, right_max = 0;
            int res = 0;
    
            while (left < right) {
                if (height[left] < height[right]) {
                    if (height[left] >= left_max) {
                        left_max = height[left];
                    } else {
                        res += left_max - height[left];
                    }
                    left++;
                } else {
                    if (height[right] >= right_max) {
                        right_max = height[right];
                    } else {
                        res += right_max - height[right];
                    }
                    right--;
                }
            }
    
            return res;
        }
    };