题目

方法

对于每一个A[i]，trap water 为 min（left， right）- A[i];

    public int trap(int[] A) {
        if (A == null || A.length == 0 || A.length == 1 || A.length == 2) {
            return 0;
        }
        int len = A.length;
        int[] left = new int[len];
        int[] right = new int[len];
        int leftMax = 0;
        for (int i = 0; i < len; i++) {
        	left[i] = leftMax;
        	if (A[i] > leftMax) {
        		leftMax = A[i];
        	}
        }
        int rightMax = 0;
        for (int i = len; i > 0; i--) {
        	right[i - 1] = rightMax;
        	if (A[i - 1] > rightMax) {
        		rightMax = A[i - 1];
        	}
        }
        
        int sum = 0;
        for (int i = 0; i < len; i++) {
        	int temp = Math.min(left[i], right[i]) - A[i];
        	if (temp > 0) {
        		sum += temp;
        	}
        }
        return sum;
    }