GFG App
Open App
Browser
Continue

Find an index of maximum occurring element with equal probability

Given an array of integers, find the most occurring element of the array and return any one of its indexes randomly with equal probability.
Examples:

Input:
arr[] = [-1, 4, 9, 7, 7, 2, 7, 3, 0, 9, 6, 5, 7, 8, 9]

Output:
Element with maximum frequency present at index 6
OR
Element with maximum frequency present at Index 3
OR
Element with maximum frequency present at index 4
OR
Element with maximum frequency present at index 12

All outputs above have equal probability.

The idea is to iterate through the array once and find out the maximum occurring element and its frequency n. Then we generate a random number r between 1 and n and finally return the r’th occurrence of maximum occurring element in the array.
Below are implementation of above idea –

C++

 // C++ program to return index of most occurring element // of the array randomly with equal probability #include #include #include using namespace std;   // Function to return index of most occurring element // of the array randomly with equal probability void findRandomIndexOfMax(int arr[], int n) {     // freq store frequency of each element in the array     unordered_map freq;     for (int i = 0; i < n; i++)         freq[arr[i]] += 1;       int max_element; // stores max occurring element       // stores count of max occurring element     int max_so_far = INT_MIN;       // traverse each pair in map and find maximum     // occurring element and its frequency     for (pair p : freq)     {         if (p.second > max_so_far)         {             max_so_far = p.second;             max_element = p.first;         }     }       // generate a random number between [1, max_so_far]     int r = (rand() % max_so_far) + 1;       // traverse array again and return index of rth     // occurrence of max element     for (int i = 0, count = 0; i < n; i++)     {         if (arr[i] == max_element)             count++;           // print index of rth occurrence of max element         if (count == r)         {             cout << "Element with maximum frequency present "                  "at index " << i << endl;             break;         }     } }   // Driver code int main() {     // input array     int arr[] = { -1, 4, 9, 7, 7, 2, 7, 3, 0, 9, 6, 5,                   7, 8, 9 };     int n = sizeof(arr) / sizeof(arr[0]);       // randomize seed     srand(time(NULL));       findRandomIndexOfMax(arr, n);       return 0; }

Java

 // Java program to return index of most occurring element // of the array randomly with equal probability import java.util.*;   class GFG {   // Function to return index of most occurring element // of the array randomly with equal probability static void findRandomIndexOfMax(int arr[], int n) {     // freq store frequency of each element in the array     HashMap mp = new HashMap();     for (int i = 0; i < n; i++)         if(mp.containsKey(arr[i]))         {             mp.put(arr[i], mp.get(arr[i]) + 1);         }         else         {             mp.put(arr[i], 1);         }       int max_element = Integer.MIN_VALUE; // stores max occurring element       // stores count of max occurring element     int max_so_far = Integer.MIN_VALUE;       // traverse each pair in map and find maximum     // occurring element and its frequency     for (Map.Entry p : mp.entrySet())     {         if (p.getValue() > max_so_far)         {             max_so_far = p.getValue();             max_element = p.getKey();         }     }           // generate a random number between [1, max_so_far]     int r = (int) ((new Random().nextInt(max_so_far) % max_so_far) + 1);       // traverse array again and return index of rth     // occurrence of max element     for (int i = 0, count = 0; i < n; i++)     {         if (arr[i] == max_element)             count++;           // print index of rth occurrence of max element         if (count == r)         {             System.out.print("Element with maximum frequency present "                 +"at index " + i +"\n");             break;         }     } }   // Driver code public static void main(String[] args) {     // input array     int arr[] = { -1, 4, 9, 7, 7, 2, 7, 3, 0, 9, 6, 5,                 7, 8, 9 };     int n = arr.length;     findRandomIndexOfMax(arr, n); } }   // This code is contributed by Rajput-Ji

Python3

 # Python3 program to return index of most occurring element # of the array randomly with equal probability import random   # Function to return index of most occurring element # of the array randomly with equal probability def findRandomIndexOfMax(arr, n):       # freq store frequency of each element in the array     mp = dict()     for i in range(n) :         if(arr[i] in mp):             mp[arr[i]] = mp[arr[i]] + 1                   else:             mp[arr[i]] = 1               max_element = -323567     # stores max occurring element       # stores count of max occurring element     max_so_far = -323567       # traverse each pair in map and find maximum     # occurring element and its frequency     for p in mp :               if (mp[p] > max_so_far):             max_so_far = mp[p]             max_element = p               # generate a random number between [1, max_so_far]     r = int( ((random.randrange(1, max_so_far, 2) % max_so_far) + 1))           i = 0     count = 0       # traverse array again and return index of rth     # occurrence of max element     while ( i < n ):               if (arr[i] == max_element):             count = count + 1           # Print index of rth occurrence of max element         if (count == r):                       print("Element with maximum frequency present at index " , i )             break         i = i + 1       # Driver code   # input array arr = [-1, 4, 9, 7, 7, 2, 7, 3, 0, 9, 6, 5, 7, 8, 9] n = len(arr) findRandomIndexOfMax(arr, n)   # This code is contributed by Arnab Kundu

C#

 using System; using System.Collections.Generic;   class GFG {   // Function to return index of most occurring element // of the array randomly with equal probability static void findRandomIndexOfMax(int[] arr, int n) {     // freq store frequency of each element in the array     Dictionary mp = new Dictionary();     for (int i = 0; i < n; i++)     {         if (mp.ContainsKey(arr[i]))         {             mp[arr[i]]++;         }         else         {             mp[arr[i]] = 1;         }     }       int max_element = int.MinValue; // stores max occurring element       // stores count of max occurring element     int max_so_far = int.MinValue;       // traverse each pair in map and find maximum     // occurring element and its frequency     foreach (KeyValuePair p in mp)     {         if (p.Value > max_so_far)         {             max_so_far = p.Value;             max_element = p.Key;         }     }       // generate a random number between [1, max_so_far]     Random rand = new Random();     int r = rand.Next(max_so_far) + 1;       // traverse array again and return index of rth     // occurrence of max element     for (int i = 0, count = 0; i < n; i++)     {         if (arr[i] == max_element)             count++;           // print index of rth occurrence of max element         if (count == r)         {             Console.WriteLine("Element with maximum frequency present "                 + "at index " + i + "\n");             break;         }     } }   // Driver code public static void Main() {     // input array     int[] arr = { -1, 4, 9, 7, 7, 2, 7, 3, 0, 9, 6, 5,                 7, 8, 9 };     int n = arr.Length;     findRandomIndexOfMax(arr, n); } }

Javascript



Output:

Element with maximum frequency present at index 4

Time complexity of above solution is O(n).
Auxiliary space used by the program is O(n).
This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

My Personal Notes arrow_drop_up