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

 1 #include <iostream>
 2 #include <vector>
 3 #include <algorithm>
 4 #include <queue>
 5 #include <stack>
 6 #include <string>
 7 #include <fstream>
 8 #include <map>
 9 using namespace std;
10 
11 int missing(int arr[], int n) {
12     for (int i = 0; i < n; i++) {
13         if (abs(arr[i]) > n) continue;
14         arr[abs(arr[i])-1] = -arr[abs(arr[i])-1];
15     }
16     for (int i = 0; i < n; i++) {
17         if (arr[i] > 0) return i + 1;
18     }
19 }
20 
21 int main() {
22     int arr[5] = {5, 4, 6, 1, 2};
23     cout << missing(arr, 5) << endl;
24     return 0;
25 }