Find the largest three elements in an array in C++

In this problem, we are given an arr[] consisting of N unsorted elements. Our task is to find the largest three elements in an array.

Let's take an example to understand the problem,

Input : arr[] = {7, 3, 9, 12, 1}
Output : 12, 9, 7

Solution Approach

We basically need to find three largest elements of the array and print them. This can be done is multiple ways,

Method 1

For the largest three elements, we will create three elements holding their values, max, max2 and max3 and set these values to arr[0].

Then we will loop form i -> 1 to n-1 and for each element

if (arr[i] > max) -> max3 = max2, max2 = max , max = arr[i].

else if (arr[i] > max2) -> max3 = max2, max2 = arr[i].

else if (arr[i] > max3) -> max3 = arr[i].

At the end of the loop, we will print all three values.

Example

Program to illustrate the working of our solution

#include <iostream>
using namespace std;
void findThreeLargestElements(int arr[], int arr_size){
   int max, max2, max3;
   max3 = max = max2 = arr[0];
   for(int i = 0; i < arr_size; i++){
      if (arr[i] > max){
         max3 = max2;
         max2 = max;
         max = arr[i];
      }
      else if (arr[i] > max2){
         max3 = max2;
         max2 = arr[i];
      }
      else if (arr[i] > max3)
         max3 = arr[i];
   }
   cout<<endl<<"Three largest elements of the array are "<<max<<", "<<max2<<", "<<max3;
}
int main(){
   int arr[] = {15, 2, 7, 86, 0, 21, 50};
   int n = sizeof(arr) / sizeof(arr[0]); 
   cout<<"The array is : ";
   for(int i = 0; i < n; i++) 
      cout<<arr[i]<<"\t"; 
   findThreeLargestElements(arr, n);
   return 0;
}

Output

The array is : 15 2 7 86 0 21 50 Three largest elements of the array are 86, 50, 21

Method 2

Another method to solve the problem is by sorting the array and then printing the first three elements of the array which are three largest elements.

Algorithm

Step 1 − sort the array using a sorting technique.

Step 2 − print first three elements : arr[0] , arr[1] , arr[2]

Example

Program to illustrate the working of our solution

#include <bits/stdc++.h>
using namespace std;
void findThreeLargestElements(int arr[], int n){
   sort(arr, arr + n, std::greater<>());
   int j = 0;
   cout<<"\nThree largest elements are ";
   for(int i = 0; i < n; i++){ 
      if(arr[i] != arr[i+1]){ 
         cout<<arr[i]<<" "; 
         j++;
      }
      if(j == 3){
         break;
      }
   }
}
int main(){
   int arr[] = {15, 2, 7, 86, 0, 21, 50, 53, 50};
   int n = sizeof(arr) / sizeof(arr[0]);
   cout<<"The array is : ";
   for(int i = 0; i < n; i++)
      cout<<arr[i]<<"\t";
   findThreeLargestElements(arr, n);
   return 0;
}

Output

The array is : 15 2 7 86 0 21 50 53 50
Three largest elements are 86 53 50