Data Structure Array: Find the Missing Number

http://www.geeksforgeeks.org/find-the-missing-number/

1. use sum formula, O(n), O(1)

2. use XOR, O(n), O(1)

3. use counting sort, O(n), O(1), changing the array

4. use negtive, O(n), O(1), chaning the array

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 using namespace std;

 int missing(int arr[], int n) {
     for (int i = ; i < n; i++) {
         if (abs(arr[i]) > n) continue;
         arr[abs(arr[i])-] = -arr[abs(arr[i])-];
     }
     for (int i = ; i < n; i++) {
         if (arr[i] > ) return i + ;
     }
 }

 int main() {
     int arr[] = {, , , , };
     cout << missing(arr, ) << endl;
     return ;
 }