Find the first repeating element in an array of integers C++

In this problem, we are an array arr of n integer values. Our task is to find the first repeating element in an array of integers.

We need to find the first integer value from the array which occurred more than once in the array.

Let's take an example to understand the problem,

Input : arr[] = {4, 1, 8, 9, 7, 2, 1, 6, 4}
Output : 4

Explanation −

Integers with more than one occurrence are 4 and 1.
4's first occurrence is smaller than 1. Hence the answer is 4

Solution Approach

A simple solution to the problem is using nested loops. We will use two loops, outer one to iterate each integer value of the array and the inner one to check if there is another integer value present in the array with the same value. If yes, return the value. This method is good but in case of no solution, it will have O(N2) complexity.

Solution Approach

Another approach which can solve the problem better is by using hashing. We will traverse the array from the last index and then update the minimum index for the element found at the index which is visited.

Example

Program to illustrate the working of our solution

#include<bits/stdc++.h>
using namespace std;
int findRepeatingElementArray(int arr[], int n){
   int minRetIndex = -1;
   set<int> visitedElements;
   for (int i = n - 1; i >= 0; i--){
      if (visitedElements.find(arr[i]) != visitedElements.end())
         minRetIndex = i;
      else
         visitedElements.insert(arr[i]);
   }
   if (minRetIndex != -1)
      return arr[minRetIndex];
   else
      return -1;
}
int main(){
   int arr[] = {4, 1, 6, 3, 4, 1, 5, 8};
   int n = sizeof(arr) / sizeof(arr[0]);
   cout<<"The element with repeated occurence is "<<findRepeatingElementArray(arr, n);
}

Output

The element with repeated occurence is 4