[brush questions] connected with rainwater (one dimension)

One 、 subject

OJ link
Given n Nonnegative integers indicate that each width is 1 Height map of columns of , Calculate columns arranged in this way , How much rain can be received after rain .

Two 、 Answer key

2.1 Ideas

2.2 Source code

 public int trap(int[] height) {
    
        if(height==null||height.length<2){
    
            return 0;
        }
        int N=height.length;
        int leftMax=height[0];
        int rightMax=height[N-1];
        int L=1;
        int R=N-2;
        int water=0;
        while(L<=R){
    
            if(leftMax<=rightMax){
    
                water+=Math.max(0,leftMax-height[L]);
                leftMax=Math.max(leftMax,height[L++]);
            }else {
    
                water+=Math.max(0,rightMax-height[R]);
                rightMax=Math.max(rightMax,height[R--]);
            }
            
        }
        return water;
    }