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(vector<int>& height) {
if(height.size() < 3) return 0;
int res = 0;
int left = 0;
int right = height.size() - 1;
int leftMax = height[0];
int rightMax = height[right];

while(left != right){
if(leftMax < rightMax){
left++;
if(height[left]<leftMax){
res +=leftMax - height[left];
}else{
leftMax = height[left];
}
}else{
right--;
if(height[right] < rightMax){
res += rightMax- height[right];
}else{
rightMax = height[right];
}
}
}
return res;
}
};