This one is simple, given an array containing all numbers from 1 to N with the exception of one print the missing number to the standard output. My solution makes use of vector. Expected complexity is O(N). As a side note, it has been said that this problem has been asked on Microsoft interviews.

#include <iostream>
#include <vector>
using namespace std;
void find_missing_number(const vector<int> &v) {
int total, i;
total = (v.size()+1)*(v.size()+2)/2;
for (i = 0; i < v.size(); i++)
total -= v[i];
cout << total;
}

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