#include <map>
#include <vector>
#include <iostream>
#include <algorithm>
 
using namespace std;
 
int lonelyinteger(const std::vector<int>& a) 
{
    typedef std::map<int, int> IntMap;
    IntMap theMap;  
 
    // build the map
    for_each(a.begin(), a.end(), [&](int n){ theMap[n]++; });
 
    // find the first entry with a count of 1
    return 
       find_if(theMap.begin(), theMap.end(),
       [](const IntMap::value_type& pr){return pr.second == 1; })->first;
}
 
int main()
{
    std::vector<int> TestVect = { 1, 1, 2 };
    cout << lonelyinteger(TestVect);
}

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