[LeetCode] Missing Number

There are three methods to solve this problem: bit manipulation, rearrangement of the array, and math tricks.

Bit Manipulation

1 class Solution {
2 public:
3     int missingNumber(vector<int>& nums) {
4         int ans = nums.size(), i = 0;
5         for (int num : nums)
6             ans ^= (num ^ (i++));
7         return ans;
8     }
9 };

Rearrangement

 1 class Solution {
 2 public:
 3     int missingNumber(vector<int>& nums) {
 4         int n = nums.size(), r = n;
 5         for (int i = 0; i < n; i++) {
 6             while (nums[i] != i) {
 7                 if (nums[i] == n) {
 8                     r = i;
 9                     break;
10                 }
11                 swap(nums[i], nums[nums[i]]);
12             }
13         }
14         return r;
15     }
16 };

Math

1 class Solution {
2 public:
3     int missingNumber(vector<int>& nums) {
4         int n = nums.size(), ans = n * (n + 1) / 2;
5         for (int num : nums) ans -= num;
6         return ans;
7     }
8 };