Maximum in array which is at-least twice of other elements

Difficulty Level : Basic
Last Updated : 09 Apr, 2021

Given an array of integers of length n. Our task is to return the index of the max element if the it is at least twice as much as every other number in the array. If the max element does not satisfy the condition return -1.
Examples:

Input : arr = {3, 6, 1, 0}
Output : 1
Here, 6 is the largest integer, and for 
every other number in the array x, 6 is 
more than twice as big as x. The index of
value 6 is 1, so we return 1.

Input : arr = {1, 2, 3, 4}
Output : -1
4 isn't at least as big as twice the value
of 3, so we return -1.

Approach : Scan through the array to find the unique largest element m, keeping track of it’s index maxIndex. Scan through the array again. If we find some x != m with m < 2*x, we should return -1. Otherwise, we should return maxIndex.

C++

// CPP program for Maximum of
// the array which is at least
// twice of other elements of
// the array.
#include<bits/stdc++.h>
using namespace std;
     
// Function to find the
// index of Max element
// that satisfies the
// condition
int findIndex(int arr[], int len) {
     
    // Finding index of
    // max of the array
    int maxIndex = 0;
    for (int i = 0; i < len; ++i)
        if (arr[i] > arr[maxIndex])
            maxIndex = i;
     
    // Returns -1 if the
    // max element is not
    // twice of the i-th
    // element.
    for (int i = 0; i < len; ++i)            
        if (maxIndex != i &&
            arr[maxIndex] < 2 * arr[i])
            return -1;
     
    return maxIndex;
}
     
// Driver function
int main(){
    int arr[] = {3, 6, 1, 0};
    int len = sizeof(arr) / sizeof(arr[0]);
     
    cout<<(findIndex(arr, len));
}
 
// This code is contributed by Smitha Dinesh Semwal

Java

// Java program for Maximum of the array
// which is at least twice of other elements
// of the array.
import java.util.*;
import java.lang.*;
 
class GfG {
     
    // Function to find the index of Max element
    // that satisfies the condition
    public static int findIndex(int[] arr) {       
         
        // Finding index of max of the array
        int maxIndex = 0;
        for (int i = 0; i < arr.length; ++i)
            if (arr[i] > arr[maxIndex])
                maxIndex = i;
         
        // Returns -1 if the max element is not
        // twice of the i-th element.       
        for (int i = 0; i < arr.length; ++i)            
            if (maxIndex != i && arr[maxIndex] < 2 * arr[i])
                return -1;
         
        return maxIndex;
    }
     
    // Driver function
    public static void main(String argc[]){
        int[] arr = new int[]{3, 6, 1, 0};
        System.out.println(findIndex(arr));
    }
}

Python3

# Python 3 program for Maximum of
# the array which is at least twice 
# of other elements of the array.
 
# Function to find the index of Max
# element that satisfies the condition
def findIndex(arr):
         
    # Finding index of max of the array
    maxIndex = 0
    for i in range(0,len(arr)):
        if (arr[i] > arr[maxIndex]):
            maxIndex = i
     
    # Returns -1 if the max element is not
    # twice of the i-th element.    
    for i in range(0,len(arr)):    
        if (maxIndex != i and
                arr[maxIndex] < (2 * arr[i])):
            return -1
         
    return maxIndex
     
     
# Driver code
arr = [3, 6, 1, 0]
print(findIndex(arr))
 
# This code is contributed by Smitha Dinesh Semwal

C#

// C# program for Maximum of the array
// which is at least twice of other elements
// of the array.
using System;
 
class GfG {
     
    // Function to find the index of Max element
    // that satisfies the condition
    public static int findIndex(int[] arr) {    
         
        // Finding index of max of the array
        int maxIndex = 0;
        for (int i = 0; i < arr.Length; ++i)
            if (arr[i] > arr[maxIndex])
                maxIndex = i;
         
        // Returns -1 if the max element is not
        // twice of the i-th element.
        for (int i = 0; i < arr.Length; ++i)        
            if (maxIndex != i && arr[maxIndex] < 2 * arr[i])
                return -1;
         
        return maxIndex;
    }
     
    // Driver function
    public static void Main()
    {
        int[] arr = new int[]{3, 6, 1, 0};
        Console.WriteLine(findIndex(arr));
    }
}
 
// This code is contributed by vt_m.

PHP

<?php
// PHP program for Maximum of
// the array which is at least
// twice of other elements of
// the array.
 
// Function to find the
// index of Max element
// that satisfies the
// condition
function findIndex($arr, $len)
{
     
    // Finding index of
    // max of the array
    $maxIndex = 0;
    for ( $i = 0; $i < $len; ++$i)
        if ($arr[$i] > $arr[$maxIndex])
            $maxIndex = $i;
     
    // Returns -1 if the
    // max element is not
    // twice of the i-th
    // element.
    for ($i = 0; $i < $len; ++$i)            
        if ($maxIndex != $i and
            $arr[$maxIndex] < 2 * $arr[$i])
            return -1;
     
    return $maxIndex;
}
     
// Driver Code
$arr = array(3, 6, 1, 0);
$len = count($arr);
 
echo findIndex($arr, $len);
 
// This code is contributed by anuj_67.
?>

Javascript

<script>
    // Javascript program for Maximum of
    // the array which is at least
    // twice of other elements of
    // the array.
     
    // Function to find the
    // index of Max element
    // that satisfies the
    // condition
    function findIndex(arr, len) {
 
        // Finding index of
        // max of the array
        let maxIndex = 0;
        for (let i = 0; i < len; ++i)
            if (arr[i] > arr[maxIndex])
                maxIndex = i;
 
        // Returns -1 if the
        // max element is not
        // twice of the i-th
        // element.
        for (let i = 0; i < len; ++i)            
            if (maxIndex != i &&
                arr[maxIndex] < 2 * arr[i])
                return -1;
 
        return maxIndex;
    }
     
    let arr = [3, 6, 1, 0];
    let len = arr.length;
       
    document.write(findIndex(arr, len));
     
    // This code is contributed by divyeshrabadiya07.
</script>

Output:

Time Complexity: $O(n)$

My Personal Notes

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