Replace every element with the smallest element on its left side

Given an array of integers, the task is to replace every element with the smallest 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 4 2 1 1
Since, 4 has no element in its left, so replace it by -1.
For 5, 4 is the smallest element in its left.
For 2, 4 is the smallest element in its left.
For 1, 2 is the smallest element in its left.
For 7, 1 is the smallest element in its left.
For 6, 1 is the smallest element in its left.

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

Approach:

1. Maintain a variable min_ele which will store the smallest element.
2. Initially, min_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 min_ele value and update the min_ele if the element is smaller than min_ele.
5. Print the modified array.

Below is the implementation of the above approach: 

C++

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

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

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

        // If min_ele is smaller than arr[i]
        // then just replace arr[i] with min_ele
        if (min_ele < arr[i])
            arr[i] = min_ele;


        // Else if update the min_ele also
        else if (min_ele >= arr[i]) {
            int temp = arr[i];
            arr[i] = min_ele;
            min_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 smaller element
// on its left side

class GFG {

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

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

            // If min_ele is smaller than arr[i]
            // then just replace arr[i] with min_ele
            if (min_ele < arr[i]) {
                arr[i] = min_ele;
            } // Else if update the min_ele also
            else if (min_ele >= arr[i]) {
                int temp = arr[i];
                arr[i] = min_ele;
                min_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.print(arr[i] + " ");
        }
    }
}

// This code is contributed by Rajput-JI

Python3

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

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

    # MIN value initialised
    # to element at 0th index
    min_ele = arr[0]
    arr[0] = -1

    for i in range(1, n):

        # If min_ele is smaller than arr[i]
        # then just replace arr[i] with min_ele
        if (min_ele < arr[i]):
            arr[i] = min_ele

        # Else if update the min_ele also
        elif (min_ele >= arr[i]) :
            temp = arr[i]
            arr[i] = min_ele
            min_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 smaller element
// on its left side
using System;
public class GFG {

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

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

            // If min_ele is smaller than arr[i]
            // then just replace arr[i] with min_ele
            if (min_ele < arr[i]) {
                arr[i] = min_ele;
            } // Else if update the min_ele also
            else if (min_ele >= arr[i]) {
                int temp = arr[i];
                arr[i] = min_ele;
                min_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 smaller element
// on its left side

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

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

        // If min_ele is smaller than arr[i]
        // then just replace arr[i] with min_ele
        if ($min_ele < $arr[$i])
            $arr[$i] = $min_ele;

        // Else if update the min_ele also
        else if ($min_ele >= $arr[$i])
        {
            $temp = $arr[$i];
            $arr[$i] = $min_ele;
            $min_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>
      // JavaScript program to Replace every
      // element with the smaller element
      // on its left side

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

        // MIN value initialised
        // to element at 0th index
        var min_ele = arr[0];
        arr[0] = -1;

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

          // If min_ele is smaller than arr[i]
          // then just replace arr[i] with min_ele
          if (min_ele < arr[i]) arr[i] = min_ele;
          // Else if update the min_ele also
          else if (min_ele >= arr[i])
          {
            var temp = arr[i];
            arr[i] = min_ele;
            min_ele = temp;
          }
        }
      }

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

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

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

      // This code is contributed by rdtank.
    </script>

Output

-1 4 4 2 1 1 

Time Complexity: O(N)

Auxiliary Space: O(1) because constant space is being used for variables