Replace every element with the greatest element on its left side

Given an array of integers, the task is to replace every element with the greatest element on its left side. 

Note: Replace the first element with -1 as it has no element in its left.

Examples:

Input: arr[] = {4, 5, 2, 1, 7, 6}
Output: -1 4 5 5 5 7
Explanation:
Since, 4 has no element in its left, so replace it by -1.
For 5, 4 is the greatest element in its left.
For 2, 5 is the greatest element in its left.
For 1, 5 is the greatest element in its left.
For 7, 5 is the greatest element in its left.
For 6, 7 is the greatest element in its left.

Input: arr[] = {3, 2, 5, 7, 1}
Output: -1 3 3 5 7

Approach:

1. Maintain a variable max_ele which will store the greatest element.
2. Initially, max_ele will be equal to the element at the 0th index.
3. First replace arr[0] with -1 then traverse the array
4. And then replace the element with the max_ele value and update the max_ele if the element is greater than max_ele.
5. Print the modified array.

Below is the implementation of the above approach: 

C++

// C++ program to Replace every
// element with the greater element
// on its left side
#include <bits/stdc++.h>
using namespace std;

// Function to replace the elements
void ReplaceElements(int arr[], int n)
{
    // Max value initialised
    // to element at 0th index
    int max_ele = arr[0];
    arr[0] = -1;

    for (int i = 1; i < n; ++i) {

        // If max_ele is greater than arr[i]
        // then just replace arr[i] with max_ele
        if (max_ele > arr[i])
            arr[i] = max_ele;

        // Else if update the max_ele also
        else if (max_ele <= arr[i]) {
            int temp = arr[i];
            arr[i] = max_ele;
            max_ele = temp;
        }
    }
}

// Driver code
int main()
{
    int arr[] = { 4, 5, 2, 1, 7, 6 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Replace the elements
    // with the smaller element
    // on its left side
    ReplaceElements(arr, n);

    // Print the modified array
    for (int i = 0; i < n; ++i)
        cout << arr[i] << " ";

    return 0;
}

Java

// Java program to Replace every
// element with the greater element
// on its left side
import java.util.*;

class Geeks {


// Function to replace the elements
static void ReplaceElements(int arr[], int n)
{
    // Max value initialised
    // to element at 0th index
    int max_ele = arr[0];
    arr[0] = -1;

    for (int i = 1; i < n; ++i) {

        // If max_ele is greater than arr[i]
        // then just replace arr[i] with max_ele
        if (max_ele > arr[i])
            arr[i] = max_ele;

        // Else if update the max_ele also
        else if (max_ele <= arr[i]) {
            int temp = arr[i];
            arr[i] = max_ele;
            max_ele = temp;
        }
    }
}

// Driver code
public static void main(String args[])
{
    int arr[] = { 4, 5, 2, 1, 7, 6 };
    int n = arr.length;

    // Replace the elements
    // with the smaller element
    // on its left side
    ReplaceElements(arr, n);

    // Print the modified array
    for (int i = 0; i < n; ++i)
        System.out.println(arr[i]);
}
}

// This code is contributed by ankita_saini

Python3

# Python3 program to Replace every
# element with the greater element
# on its left side

# Function to replace the elements
def ReplaceElements(arr, n):

    # Max value initialised
    # to element at 0th index
    max_ele = arr[0]
    arr[0] = -1

    for i in range(1, n):

        # If max_ele is greater than arr[i]
        # then just replace arr[i] with max_ele
        if (max_ele > arr[i]):
            arr[i] = max_ele

        # Else if update the max_ele also
        elif (max_ele <= arr[i]):
            temp = arr[i]
            arr[i] = max_ele
            max_ele = temp

# Driver code
if __name__ == "__main__":

    arr = [4, 5, 2, 1, 7, 6 ]
    n = len(arr)

    # Replace the elements
    # with the smaller element
    # on its left side
    ReplaceElements(arr, n)

    # Print the modified array
    for i in range (n):
        print( arr[i], end = " ")

# This code is contributed
# by ChitraNayal

C#

// C# program to Replace every
// element with the greater element
// on its left side
using System;
public class GFG {

// Function to replace the elements
static void ReplaceElements(int []arr, int n)
{
    // Max value initialised
    // to element at 0th index
    int max_ele = arr[0];
    arr[0] = -1;

    for (int i = 1; i < n; ++i) {

        // If max_ele is greater than arr[i]
        // then just replace arr[i] with max_ele
        if (max_ele > arr[i])
            arr[i] = max_ele;

        // Else if update the max_ele also
        else if (max_ele <= arr[i]) {
            int temp = arr[i];
            arr[i] = max_ele;
            max_ele = temp;
        }
    }
}

// Driver code
public static void Main()
{
    int []arr = { 4, 5, 2, 1, 7, 6 };
    int n = arr.Length;

    // Replace the elements
    // with the smaller element
    // on its left side
    ReplaceElements(arr, n);

    // Print the modified array
    for (int i = 0; i < n; ++i) {
        Console.Write(arr[i]+" ");
    }
}
}

// This code is contributed by Rajput-Ji

PHP

<?php
// PHP program to Replace every
// element with the greater element
// on its left side

// Function to replace the elements
function ReplaceElements($arr, $n)
{
    // Max value initialised
    // to element at 0th index
    $max_ele = $arr[0];
    $arr[0] = -1;

    for ($i = 1; $i < $n; ++$i)
    {

        // If max_ele is greater than arr[i]
        // then just replace arr[i] with max_ele
        if ($max_ele > $arr[$i])
            $arr[$i] = $max_ele;

        // Else if update the max_ele also
        else if ($max_ele <= $arr[$i])
        {
            $temp = $arr[$i];
            $arr[$i] = $max_ele;
            $max_ele = $temp;
        }
    }
    return $arr;
}

// Driver code
$arr = array(4, 5, 2, 1, 7, 6);
$n = sizeof($arr);

// Replace the elements
// with the smaller element
// on its left side
$arr1 = ReplaceElements($arr, $n);

// Print the modified array
for ($i = 0; $i < $n; ++$i)
    echo $arr1[$i] . " ";

// This code is contributed
// by Akanksha Rai

Javascript

<script>
// Java script program to Replace every
// element with the greater element
// on its left side

// Function to replace the elements
function ReplaceElements(arr,n)
{

    // Max value initialised
    // to element at 0th index
    let max_ele = arr[0];
    arr[0] = -1;

    for (let i = 1; i < n; ++i) {

        // If max_ele is greater than arr[i]
        // then just replace arr[i] with max_ele
        if (max_ele > arr[i])
            arr[i] = max_ele;

        // Else if update the max_ele also
        else if (max_ele <= arr[i]) {
            let temp = arr[i];
            arr[i] = max_ele;
            max_ele = temp;
        }
    }
}

// Driver code
    let arr = [ 4, 5, 2, 1, 7, 6 ];
    let n = arr.length;

    // Replace the elements
    // with the smaller element
    // on its left side
    ReplaceElements(arr, n);

    // Print the modified array
    for (let i = 0; i < n; ++i)
        document.write(arr[i]+" ");

// This code is contributed by sravan kumar G
</script>

Output

-1 4 5 5 5 7 

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