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 = 0; i < n; i++) {
        if (abs(arr[i]) > n) continue;
        arr[abs(arr[i])-1] = -arr[abs(arr[i])-1];
    }
    for (int i = 0; i < n; i++) {
        if (arr[i] > 0) return i + 1;
    }
}

int main() {
    int arr[5] = {5, 4, 6, 1, 2};
    cout << missing(arr, 5) << endl;
    return 0;
}