找到数组中唯一重复的数

#include<iostream>
#include<algorithm>
#include<numeric>
using namespace std;

int helper1(int a[],int n)
{
  int sum = accumulate(a,a+n,0);
  int sum2 = n*(n-1)/2;
  return sum-sum2;
}

int helper2(int a[],int n)
{
  int bitor = a[0]^0;
  for(int i = 1;i< n;i++)
  {
      bitor ^= a[i];
      bitor ^= i;
  }
  return bitor;
}

int main()
{
    int a[] = {1,2,3,4,5,5};
    cout<<helper1(a,6)<<endl;
    cout<<helper2(a,6)<<endl;
}