LeetCode - Trapping Rain Water

Trapping Rain Water

//
public int trap(int[] height) {
        if(height == null || height.length <= 2) return 0;
        int left = 0, right = height.length - 1, sum = 0, h = 0;
        while(left < right){
            if(height[left] <= height[right]){
                if(height[left] < h){
                    sum += h - height[left];
                }
                h = Math.max(h, height[left]);
                left++;
            } else {
                if(height[right] < h){
                    sum += h - height[right];
                }
                h = Math.max(h, height[right]);
                right--;
            }
        }
        return sum;
    }
    
//
public int trap(int[] A) {
        //Solution 2: Scan only one time
        if(A==null || A.length<=2) return 0;
        int left=0, right = A.length-1, leftMax=0, rightMax=0, max=0;
        while(left<=right){
            //move left bar toward right
            if(leftMax<=rightMax){
                if(leftMax-A[left]>0)
                    max += leftMax-A[left];
                leftMax = Math.max(leftMax, A[left]);
                left++;
            }else{
                //move right bar toward left
                if(rightMax-A[right]>0)
                    max += rightMax-A[right];
                rightMax = Math.max(rightMax, A[right]);
                right--;
            }
        }
        return max;
    }
    
//
public int trap(int[] A) {
        //Solution 1: scan twice
        if(A==null || A.length<3) return 0;
        int leftMax=A[0], rightMax=A[A.length-1], max=0;
        int[] ret = new int[A.length];
        for(int i=1; i<A.length-1; i++){
            ret[i] = leftMax;
            leftMax = Math.max(leftMax, A[i]);
        }
        for(int i=A.length-2; i>0; i--){
            if(Math.min(rightMax, ret[i])-A[i]>0){
                max += Math.min(rightMax, ret[i]) - A[i];
            }
            rightMax = Math.max(rightMax, A[i]);
        }
        return max;
    }