66. Plus One

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.

从后往前遍历数组，如果当前元素<9的，当前元素加一，并直接返回数组；如果当前元素>=9(=9)，说明需要进位，把当前元素设为0。遍历完全部元素还没有遇到<9的元素，说明需要增加位数，返回一个新数组 1, 000...000

时间：O(N)，空间：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;
    }
}