Maximum Fixed Point (Value equal to index) in a given Array

Last Updated : 18 Jul, 2021
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Given an array arr[] of size N, the task is to find the maximum index i such that arr[i] is equal to i. If there is no such index in the array arr[] then print -1.

Examples:

Input: arr[ ] = {-10, -5, 0, 3, 7}
Output: 3
Explanation: Only for i=3, arr[3] = 3

Input: arr[ ] = {0, 2, 5, 8, 4}
Output: 4

Approach: Follow the steps below to solve this problem:

  • Iterate in the range [N-1, 0] using the variable i: 
    • If the current element is equal to i, then print i and return.
  • If there is no such index, then print -1.

Below is the implementation of the above approach:

C++ 
// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to  find the maximum index
// i such that arr[i] is equal to i
void findLargestIndex(int arr[], int n)
{

    // Traversing the array from
    // backwards
    for (int i = n - 1; i >= 0; i--) {

        // If arr[i] is equal to i
        if (arr[i] == i) {
            cout << i << endl;
            return;
        }
    }

    // If there is no such index
    cout << -1 << endl;
}

// Driver code
int main()
{
    // Given Input
    int arr[] = { -10, -5, 0, 3, 7 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Function Call
    findLargestIndex(arr, n);
    return 0;
}
Java 
// Java implementation of the above approach
import java.io.*;

class GFG
{
  
    // Function to  find the maximum index
    // i such that arr[i] is equal to i
    static void findLargestIndex(int arr[], int n)
    {

        // Traversing the array from
        // backwards
        for (int i = n - 1; i >= 0; i--) {

            // If arr[i] is equal to i
            if (arr[i] == i) {
                System.out.println(i);
                return;
            }
        }

        // If there is no such index
        System.out.println(-1);
    }

    // Driver code
    public static void main(String[] args)
    {
      
        // Given Input
        int arr[] = { -10, -5, 0, 3, 7 };
        int n = arr.length;

        // Function Call
        findLargestIndex(arr, n);
    }
}

// This code is contributed by Potta Lokesh
Python 
# Python implementation of the above approach
# Function to  find the maximum index
# i such that arr[i] is equal to i
def findLargestIndex(arr,  n):

    # Traversing the array from
    # backwards
    for i in range (n):

        # If arr[i] is equal to i
        if (arr[i] == i) :
            print( i )
            return
        
    # If there is no such index
    print( -1 )

# Driver code
# Given Input
arr =  [-10, -5, 0, 3, 7 ]
n = len(arr)

# Function Call
findLargestIndex(arr, n)
    
# This code is contributed by shivanisinghss2110
C# 
// C# implementation of the above approach
using System;

class GFG
{
  
    // Function to  find the maximum index
    // i such that arr[i] is equal to i
    static void findLargestIndex(int []arr, int n)
    {

        // Traversing the array from
        // backwards
        for (int i = n - 1; i >= 0; i--) {

            // If arr[i] is equal to i
            if (arr[i] == i) {
                Console.Write(i);
                return;
            }
        }

        // If there is no such index
        Console.Write(-1);
    }

    // Driver code
    public static void Main(String[] args)
    {
      
        // Given Input
        int []arr = { -10, -5, 0, 3, 7 };
        int n = arr.Length;

        // Function Call
        findLargestIndex(arr, n);
    }
}

// This code is contributed by shivanisinghss2110
JavaScript 
<script>
// JavaScript implementation of the above approach

// Function to  find the maximum index
// i such that arr[i] is equal to i
function findLargestIndex( arr,  n)
    {

        // Traversing the array from
        // backwards
        for (var i = n - 1; i >= 0; i--) {

            // If arr[i] is equal to i
            if (arr[i] == i) {
                document.write(i);
                return ;
            }
        }

        // If there is no such index
        document.write(-1);
    }

    // Driver code
    // Given Input
        var arr = [ -10, -5, 0, 3, 7 ];
        var n = arr.length;

        // Function Call
        findLargestIndex(arr, n);

// This code is contributed by shivanisinghss2110
</script>

 
 


Output
3


 

Time Complexity: O(N)
Auxiliary Space: O(1)


 


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