Given a non-empty array of digits representing a non-negative integer, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit.

You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:

Input: [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.

Example 2:

Input: [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.

從後往前遍歷陣列，如果當前元素<9的，當前元素加一，並直接返回陣列；如果當前元素>=9(=9)，說明需要進位，把當前元素設為0。遍歷完全部元素還沒有遇到<9的元素，說明需要增加位數，返回一個新陣列 1, 000...000

時間：O(N)，空間：O(1)

class Solution {
    public int[] plusOne(int[] digits) {
        for(int i = digits.length - 1; i >= 0; i--) {
            if(digits[i] < 9) {
                digits[i] += 1;
                return digits;
            }
            digits[i] = 0;   
        }
        int[] res = new
 
 int[digits.length + 1];
        res[0] = 1;
        return res;
    }
}