题目：

https://leetcode.com/problems/plus-one/
Given a non-negative number represented as an array of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

题目虽简单，但是要注意的是当最高位是9的时候， 需要重新new一个长一位的新数组来存储结果。

public class Solution {  
    public int[] plusOne(int[] digits) {  
        if (digits == null || digits.length == 0) return digits;  
        int carry = 1;  
        for (int i=digits.length - 1; i >= 0; i--) {  
            digits[i] = digits[i] + carry;  
            if (digits[i] < 10) {  
                return digits;  
            } else {  
                digits[i] = 0;  
                carry = 1;  
            }  
        }  
        int[] newDigits = new int[digits.length + 1];  
        newDigits[0] = 1;  
        System.arraycopy(digits, 0, newDigits, 1, digits.length);  
        return newDigits;  
    }  
}