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 }