leetcode-42 Trapping Rain Water

问题描述：

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

For example, 

Given 

[0,1,0,2,1,0,1,3,2,1,2,1]

, return 

6

.



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!

 

问题分析：

同样也是双指针法解决问题，类似的问题有：《leetcode-11 Container With Most Water》

找到左右边界，然后逐步向里逼近；时间复杂度O(n)

 

代码：

public class Solution {
public int trap(int[] A) {
// 双指针法，记录左右指针位置
int left = 0, right = A.length - 1;
int result = 0;
// 记录左右边界的最高高度
int maxLeft = 0, maxRight = 0;

while (left <= right) {
// 由于水桶效应，故与left,right较小值相关
if (maxLeft <= maxRight) {
if (A[left] > maxLeft)
maxLeft = A[left];
else
result += (maxLeft -A[left]);
left++;
} else {
if (A[right] > maxRight)
maxRight = A[right];
else
result += (maxRight -A[right]);
right--;
}
}
return result;
}
}