Trapping Rain Water

Trapping Rain Water

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.

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!

Java代码：

public class Solution {    public int trap(int[] A) {         int answer = 0;    if (A.length <= 2) {        answer = 0;    }    else {        int maxValue = 0;        int maxValueIndex = 0;        for (int i = 0; i < A.length; i++) {            int elem = A[i];            if (elem > maxValue) {                maxValue = elem;                maxValueIndex = i;            }        }        int totalDrops = 0;        int leftMax = 0;        for (int i = 0; i < maxValueIndex; i++) {            int elem = A[i];            if (elem > leftMax) {                leftMax = elem;            }            else {                totalDrops += leftMax - A[i];            }        }        int rightMax = 0;        for (int i = A.length - 1; i > maxValueIndex; i--) {            int elem = A[i];            if (elem > rightMax) {                rightMax = elem;            }            else {                totalDrops += rightMax - A[i];            }        }        answer = totalDrops;    }    return answer;    }}