268. Missing Number

O(n) time O(1) space
利用下标从0到n，把所有数异或最后剩下的就是差的数

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int n = nums.size();
        int res = n;
        for (int i = 0; i < n; ++i) {
            res ^= (i ^ nums[i]);
        }
        return res;
    }
};