Trapping Rain Water

Created: sep 6 2018, 11:32, Updated 09/26/2021

LeetCode

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

Example:

Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

Basic Idea

Solution 1： Two Pointers

对于一对左右边界来说，如果不考虑之前可能有更高的边界，这两个边界中间水位高度取决于较低的边界。于是，我们可以用两个指针分别从两边出发向中间逼近，每次更新leftMax，rightMax，waterHeight 和 water。具体方法是，如果 leftMax > rightMax, 则 right 向左逼近，否则 left 向右逼近。

• Java Code:

  // Updated: 09/26/2021
  class Solution {
      public int trap(int[] height) {
          int left = 0, right = height.length - 1;
          int leftMax = height[left];
          int rightMax = height[right];
          int water = 0;
          while (left < right) {
              if (leftMax > rightMax) {
                  // right位置的水量就是水面高度减去地面高度
                  water += rightMax - height[right];
                  right--;
                  rightMax = Math.max(rightMax, height[right]);
              } else {
                  water += leftMax - height[left];
                  left++;
                  leftMax = Math.max(leftMax, height[left]);
              }
          }
          return water;
      }
  }

Solution 2: DP

对于每个横坐标，我们可以通过检查其左边的最高墙 leftMax 和其右边的最高墙 rightMax 求得该位置水的高度，我们只要扫描一遍横坐标，将每个位置上水位高度加起来就可以了。

那么如何用 O(n) 时间求得每个点左边的最大值 leftMax 和右边最大值 rightMax 呢，我们可以利用dp的思路先生成这两个数组，总时间复杂度为 O(3n);

Java Code:

class Solution {
    public int trap(int[] height) {
        if (height.length == 0) return 0;
        int[] leftMaxs = new int[height.length];
        int[] rightMaxs = new int[height.length];
        // 生成leftMax数组
        leftMaxs[0] = height[0];
        for (int i = 1; i < height.length; ++i) {
            if (leftMaxs[i - 1] < height[i]) {
                leftMaxs[i] = height[i];
            } else {
                leftMaxs[i] = leftMaxs[i - 1];
            }
        }
        // 生成rightMax数组
        rightMaxs[rightMaxs.length - 1] = height[height.length - 1];
        for (int i = height.length - 2; i >= 0; --i) {
            if (rightMaxs[i + 1] < height[i]) {
                rightMaxs[i] = height[i];
            } else {
                rightMaxs[i] = rightMaxs[i + 1];
            }
        }
        // 扫描height数组，累加每一条water
        int water = 0;
        for (int i = 0; i < height.length; ++i) {
            int waterHeight = Math.min(leftMaxs[i], rightMaxs[i]);
            if (waterHeight > height[i]) {
                water += waterHeight - height[i];
            }
        }
        return water;
    }
}