leetcode[42]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!

class Solution {
public:
int trap(int A[], int n) {
if (A==NULL||n<3)
return 0;
int maxleft=0;
int maxright=0;
int i=0,j=n-1;
int sum=0;
while(i<j)
{
if (maxleft<A[i])
{
maxleft=A[i];
}
if (maxright<A[j])
{
maxright=A[j];
}
if (maxleft<=maxright)
{
sum+=maxleft-A[i];
i++;
}
else
{
sum+=maxright-A[j];
j--;
}
}
return sum;
}
};