1
class Solution {
2
  public int trap(int[] height) {
3
    int left = 0;
4
    int right = height.length - 1;
5

6
    int l_max = height[left];
7
    int r_max = height[right];
8
    int res = 0;
9

10
    while (left < right) {
11
      if (l_max < r_max) {
12
        left += 1;
13
        l_max = Math.max(l_max, height[left]);
14
        res += l_max - height[left];
15
      } else {
16
        right -= 1;
17
        r_max = Math.max(r_max, height[right]);
18
        res += r_max - height[right];
19
      }
20
    }
21

22
    return res;
23
  }
24
}