class Solution {
    public int trap(int[] height) {
        int ans = 0;
        int left = 0;
        int right = height.length-1;

        int leftMax = 0;
        int rightMax = 0;
        while(left < right) {
            if(height[left] < height[right]) {
                if(height[left] >= leftMax) {
                    leftMax = height[left];
                } else {
                    ans += leftMax - height[left];
                }
                left += 1;
            } else {
                if(height[right] >= rightMax) {
                    rightMax = height[right];
                } else {
                    ans += rightMax - height[right];
                }
                right -= 1;
            }
        }
        return ans;
    }
}