XORTRICK

/* suppose the problem says tha we are given an array with only one unique element and rest all elements have their duplicates and we have to find the 
unique element in O(n) time and O(1) space complexity then we can use the idea of XOR as shown below:
*/
#include<iostream>
using namespace std;
int main(){
int ans=0;
  for(int i=0;i<n;++i)
  ans ^= arr[i];
  // this will find the unique element in the array in O(1) space complexity
 return ans;
}