Find the non duplicate element

There is an array in which except one element, all other elements have duplicates, find that non duplicate element.

Answer:

/**
 * An array in which all elements except one are duplicate.,
 * find that element
 * */
#include "vector"
#include "algorithm"
#include "stdio.h"
using namespace std;

/** SOLUTION I 
 * Sort the array - O(n log(n))
 * Find the duplicate - O(n) (single pass)
 * */
void SortAndFind(vector &vec)
{
    // O(n log(n))
    sort(vec.begin(), vec.end() );
    int i = 0;
    int j = i + 1;
    int n = vec.size();
    // O(n)
    for ( i = 0, j = i+1; j < n ; ++j)
    {
        if ( vec[j] != vec[i] )
        {
            if ( (j-i) == 1 )
            {
                /* non duplicate found at j */
                printf("\n Non Duplicate (%s) %d \n", __func__, vec[i]);
                return;
            }
            else
            {
                i=j;
            }
        }
    }
    // the last element is the duplicate
    printf("\n Non Duplicate (%s) %d \n", __func__, vec[n-1]);
}

/** SOLUTION II , better than first solution.
 * Heapify the array - O(n)
 * Remove one element at a time and find the duplicate - O(log(n))
 * This approach is better because there is no need to sort the entire array
 * */
void HeapifyAndFind(vector &vec)
{
    // Create the heap O(n)
    make_heap(vec.begin(), vec.end() );
    // Find the duplicate, by removing one element at a
    // time.
    //
    int i = 0;
    int n = vec.size();
    for ( int j = i+1; j < n; ++j)
    {
        // vec[j-1] is the min element
        make_heap( vec.begin() + i , vec.end() );
        if ( vec[i] != vec[j] )
        {
            if ( (j-i) == 1 )
            {
                /* non duplicate found at j */
                printf("\n Non Duplicate (%s) %d \n", __func__, vec[i]);
                return;
            }
            else
            {
                i=j;
            }
        }

    }
    // the last element is the duplicate
    printf("\n Non Duplicate (%s) %d \n", __func__, vec[n-1]);

}

int main(int argc, char *argv[])
{
    if ( argc > 1 )
    {
        vector vec1(argc-1);
        vector vec2(argc-1);

        for (int i = 1; i <= argc-1; ++i)
        {
            vec1[i-1] = atoi(argv[i]);
            vec2[i-1] = atoi(argv[i]);
        }

        SortAndFind( vec1 );
        HeapifyAndFind( vec2 );
    }
    return 0;
}