Missing Number

https://leetcode.com/problems/missing-number/

Given an array containing n distinct numbers taken from 0, 1, 2, …, n, find the one that is missing from the array.

Example 1:

Input: [3,0,1]
Output: 2

Example 2:

Input: [9,6,4,2,3,5,7,0,1]
Output: 8

Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int ret;
        int n = nums.size();
        int checkout[n + 1];
        for (int i = 0; i < n + 1; ++i) {
            checkout[i] = 0;
        }
        for (int i = 0; i < n; ++i) {
            checkout[nums[i]] = 1;
        }
        for (int i = 0; i < n + 1; ++i) {
            if (!checkout[i]) {
                ret = i;
                break;
            }
        }
        return ret;
    }
};

class Solution {
    public int missingNumber(int[] nums) {
        int ret = 0;
        int n = nums.length;
        int[] checkout = new int[n + 1];
        for (int i = 0; i < n + 1; ++i) {
            checkout[i] = 0;
        }
        for (int i = 0; i < n; ++i) {
            checkout[nums[i]] = 1;
        }
        for (int i = 0; i < n + 1; ++i) {
            if (checkout[i] == 0) {
                ret = i;
                break;
            }
        }
        return ret;
    }
}