题解 | 缺失数字

class Solution {
public:
    int solve(vector& a) {
        int len = a.size();
        for(int i = 0; i < len; i++)
        {
            if(i == 0 && a[i] != 0)
            {
                return 0;
            }
            else if(i == len - 1 && a[i] != len)
            {
                return len;
            }
            else if(a[i + 1] != a[i] + 1)
            {
                return a[i] + 1;
            }
        }
        return 0;
    }
};