[leetcode]42. Trapping Rain Water(Java)

https://leetcode.com/problems/trapping-rain-water/#/description

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 Marcosfor contributing this image!

Java code:

package go.jacob.day623;

import org.junit.Test;

public class Demo1 {

@Test
public void testName() throws Exception {
int[] num = { 0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1 };
System.out.println(trap(num));
}
/*
* Runtime: 22 ms
* 参考自leetcode网友：@yuyibestman
*/
public int trap(int[] height) {
if (height == null || height.length < 2)
return 0;
int leftMax = height[0], rightMax = height[height.length - 1], left = 0, right = height.length - 1;

int sum = 0;
while (left < right) {
leftMax = Math.max(height[left], leftMax);
rightMax = Math.max(height[right], rightMax);
if (leftMax < rightMax) {
sum += leftMax - height[left];
left++;
} else {
sum += rightMax - height[right];
right--;
}

}

return sum;
}

/*
* 用总面积减去空白面积，再减去边的面积：分别从两侧从中间搜索，找到最高的边停止 runtime:21ms
*/
public int trap_1(int[] height) {
if (height == null || height.length < 2)
return 0;
int max = height[0];
int sumOfNum = 0;
// 记录空白面积
int additionalArea = 0;

// 找到最高边和所有边的和
for (int i = 0; i < height.length; i++) {
if (max < height[i])
max = height[i];
sumOfNum += height[i];
}

int tmpLarge = height[0];
int otherEdge = 0;
// 从左边开始搜索
for (int i = 0; i < height.length; i++) {
if (tmpLarge < height[i]) {
additionalArea += (i - otherEdge) * (max - tmpLarge);
otherEdge = i;
tmpLarge = height[i];
}
// 找到最高边，停止搜索
if (height[i] == max)
break;
}

tmpLarge = height[height.length - 1];
otherEdge = height.length - 1;
// 从右边开始搜索
for (int i = height.length - 1; i >= 0; i--) {
if (tmpLarge < height[i]) {
additionalArea += (otherEdge - i) * (max - tmpLarge);
tmpLarge = height[i];
otherEdge = i;
}
// 找到最高边，停止搜索
if (height[i] == max)
break;
}
int area = height.length * max - sumOfNum - additionalArea;
return area;
}
}