Replace every array element by sum of previous and next

12 May, 2021
Given an array of integers, update every element with sum of previous and next elements with following exceptions. a) First element is replaced by sum of first and second. b) Last element is replaced by sum of last and second last.Examples:  
Input : arr[] = { 2, 3, 4, 5, 6}
Output : 5 6 8 10 11
Explanation: We get the following array as {2+3, 2+4, 3+5, 4+6, 5+6}

Input : arr[] = { 3, 2, 1}
Output : 5 4 3
 
A Simple Solution is to create an auxiliary array, copy contents of given array to auxiliary array. Finally traverse the auxiliary array and update given array using copied values. Time complexity of this solution is O(n), but it requires O(n) extra space.An efficient solution can solve the problem in O(n) time and O(1) space. The idea is to keep track of previous element in loop. Add the previous element using the extra variable and the next element to get each element.Below is the implementation of this idea.  
C++
Java
Python 3
C#
PHP
Javascript
// C++ program to update every array element with// sum of previous and next numbers in array#include <iostream>using namespace std; void ReplaceElements(int arr[], int n){    // Nothing to do when array size is 1    if (n <= 1)        return;     // store current value of arr[0] and update it    int prev = arr[0];    arr[0] = arr[0] + arr[1];     // Update rest of the array elements    for (int i = 1; i < n - 1; i++) {         // Store current value of next iteration        int curr = arr[i];         // Update current value using previews value        arr[i] = prev + arr[i + 1];         // Update previous value        prev = curr;    }     // Update last array element separately    arr[n - 1] = prev + arr[n - 1];} // Driver programint main(){    int arr[] = { 2, 3, 4, 5, 6 };    int n = sizeof(arr) / sizeof(arr[0]);     ReplaceElements(arr, n);     // Print the modified array    for (int i = 0; i < n; i++)        cout << arr[i] << " ";    return 0;}
// Java program to update every array element with// sum of previous and next numbers in array import java.io.*; class GFG {        static void ReplaceElements(int arr[], int n) {        // Nothing to do when array size is 1        if (n <= 1) {            return;        }         // store current value of arr[0] and update it        int prev = arr[0];        arr[0] = arr[0] + arr[1];         // Update rest of the array elements        for (int i = 1; i < n - 1; i++) {             // Store current value of next iteration            int curr = arr[i];             // Update current value using previews value            arr[i] = prev + arr[i + 1];             // Update previous value            prev = curr;        }         // Update last array element separately        arr[n - 1] = prev + arr[n - 1];    } // Driver program         public static void main (String[] args) {         int arr[] = {2, 3, 4, 5, 6};        int n = arr.length;        ReplaceElements(arr, n);        // Print the modified array        for (int i = 0; i < n; i++) {            System.out.print(arr[i] + " ");        }    }}// This code is contributed by akt_mit
# Python 3 program to update every array# element with sum of previous and next# numbers in array def ReplaceElements(arr, n):     # Nothing to do when array size is 1    if (n <= 1):        return     # store current value of arr[0]    # and update it    prev = arr[0]    arr[0] = arr[0] + arr[1]     # Update rest of the array elements    for i in range(1, n - 1):         # Store current value of        # next iteration        curr = arr[i]         # Update current value using        # previews value        arr[i] = prev + arr[i + 1]         # Update previous value        prev = curr     # Update last array element separately    arr[n - 1] = prev + arr[n - 1] # Driver Codeif __name__ == "__main__":     arr = [ 2, 3, 4, 5, 6 ]    n = len(arr)     ReplaceElements(arr, n)     # Print the modified array    for i in range(n):        print (arr[i], end = " ") # This code is contributed# by ChitraNayal
// C# program to update every array element with// sum of previous and next numbers in arrayusing System;public class GFG {     static void ReplaceElements(int []arr, int n) {        // Nothing to do when array size is 1        if (n <= 1) {            return;        }         // store current value of arr[0] and update it        int prev = arr[0];        arr[0] = arr[0] + arr[1];         // Update rest of the array elements        for (int i = 1; i < n - 1; i++) {             // Store current value of next iteration            int curr = arr[i];             // Update current value using previews value            arr[i] = prev + arr[i + 1];             // Update previous value            prev = curr;        }         // Update last array element separately        arr[n - 1] = prev + arr[n - 1];    } // Driver program    public static void Main() {        int []arr = {2, 3, 4, 5, 6};        int n = arr.Length;         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 program to update every array// element with sum of previous and// next numbers in array function ReplaceElements($arr, $n){    // Nothing to do when array    // size is 1    if ($n <= 1)        return;     // store current value of    // arr[0] and update it    $prev = $arr[0];    $arr[0] = $arr[0] + $arr[1];     // Update rest of the array elements    for ($i = 1; $i < $n - 1; $i++)    {         // Store current value of        // next iteration        $curr = $arr[$i];         // Update current value using        // previews value        $arr[$i] = $prev + $arr[$i + 1];         // Update previous value        $prev = $curr;    }     // Update last array element    // separately    $arr[$n - 1] = $prev + $arr[$n - 1];    return $arr;} // Driver Code$arr = array(2, 3, 4, 5, 6);$n = sizeof($arr); $arr1 = ReplaceElements($arr, $n); // Print the modified arrayfor ($i = 0; $i < $n; $i++)    echo $arr1[$i] . " "; // This code is contributed// by Akanksha Rai?>
<script>    // Javascript program to update every array element with    // sum of previous and next numbers in array         function ReplaceElements(arr, n) {        // Nothing to do when array size is 1        if (n <= 1) {            return;        }           // store current value of arr[0] and update it        let prev = arr[0];        arr[0] = arr[0] + arr[1];           // Update rest of the array elements        for (let i = 1; i < n - 1; i++) {               // Store current value of next iteration            let curr = arr[i];               // Update current value using previews value            arr[i] = prev + arr[i + 1];               // Update previous value            prev = curr;        }           // Update last array element separately        arr[n - 1] = prev + arr[n - 1];    }         let arr = [2, 3, 4, 5, 6];    let n = arr.length;     ReplaceElements(arr, n);     // Print the modified array    for (let i = 0; i < n; i++) {      document.write(arr[i] + " ");    } </script>
5 6 8 10 11
 
Time Complexity – O(N) 
Rajput-Ji
jit_t
ukasp
Akanksha_Rai
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