public:

int missingNumber(vector<int>& nums) {

int num = 0;

for (int i = 0; i < nums.size(); ++i) {

num ^= nums[i] ^ (i + 1);

}

return num;

}

public:

int missingNumber(vector<int>& nums) {

vector<int> expected(nums.size());

iota(expected.begin(), expected.end(), 1); // Costs extra space O(n)

return accumulate(nums.cbegin(), nums.cend(), 0, bit_xor<int>()) ^

accumulate(expected.cbegin(), expected.cend(), 0, bit_xor<int>());

}