Maximum sum such that no two elements are adjacent

Difficulty Level : Medium
Last Updated : 18 May, 2021

Given an array of positive numbers, find the maximum sum of a subsequence with the constraint that no 2 numbers in the sequence should be adjacent in the array. So 3 2 7 10 should return 13 (sum of 3 and 10) or 3 2 5 10 7 should return 15 (sum of 3, 5 and 7).Answer the question in most efficient way.

Examples :

Input : arr[] = {5, 5, 10, 100, 10, 5}
Output : 110

Input : arr[] = {1, 2, 3}
Output : 4

Input : arr[] = {1, 20, 3}
Output : 20

Algorithm:
Loop for all elements in arr[] and maintain two sums incl and excl where incl = Max sum including the previous element and excl = Max sum excluding the previous element.
Max sum excluding the current element will be max(incl, excl) and max sum including the current element will be excl + current element (Note that only excl is considered because elements cannot be adjacent).
At the end of the loop return max of incl and excl.

Example:

  arr[] = {5,  5, 10, 40, 50, 35}

  incl = 5 
  excl = 0

  For i = 1 (current element is 5)
  incl =  (excl + arr[i])  = 5
  excl =  max(5, 0) = 5

  For i = 2 (current element is 10)
  incl =  (excl + arr[i]) = 15
  excl =  max(5, 5) = 5

  For i = 3 (current element is 40)
  incl = (excl + arr[i]) = 45
  excl = max(5, 15) = 15

  For i = 4 (current element is 50)
  incl = (excl + arr[i]) = 65
  excl =  max(45, 15) = 45

  For i = 5 (current element is 35)
  incl =  (excl + arr[i]) = 80
  excl =  max(65, 45) = 65

And 35 is the last element. So, answer is max(incl, excl) =  80

Thanks to Debanjan for providing code.

Implementation:

C++

//c++ program for the above approach
#include <bits/stdc++.h>
 
using namespace std;
 
 
/*Function to return max sum such that no two elements
  are adjacent */
int FindMaxSum(vector<int> arr, int n)
{
    int incl = arr[0];
    int excl = 0;
    int excl_new;
    int i;
 
    for (i = 1; i < n; i++)
    {
        /* current max excluding i */
        excl_new = (incl > excl) ? incl : excl;
 
        /* current max including i */
        incl = excl + arr[i];
        excl = excl_new;
    }
 
    /* return max of incl and excl */
    return ((incl > excl) ? incl : excl);
}
 
// Driver program to test above functions
int main()
{
    vector<int> arr = {5, 5, 10, 100, 10, 5};
    cout<<FindMaxSum(arr, arr.size());
}

C

#include<stdio.h>
 
/*Function to return max sum such that no two elements
are adjacent */
int FindMaxSum(int arr[], int n)
{
int incl = arr[0];
int excl = 0;
int excl_new;
int i;
 
for (i = 1; i < n; i++)
{
    /* current max excluding i */
    excl_new = (incl > excl)? incl: excl;
 
    /* current max including i */
    incl = excl + arr[i];
    excl = excl_new;
}
 
/* return max of incl and excl */
return ((incl > excl)? incl : excl);
}
 
/* Driver program to test above function */
int main()
{
int arr[] = {5, 5, 10, 100, 10, 5};
int n = sizeof(arr) / sizeof(arr[0]);
printf("%d n", FindMaxSum(arr, n));
return 0;
}

Java

class MaximumSum
{
    /*Function to return max sum such that no two elements
      are adjacent */
    int FindMaxSum(int arr[], int n)
    {
        int incl = arr[0];
        int excl = 0;
        int excl_new;
        int i;
 
        for (i = 1; i < n; i++)
        {
            /* current max excluding i */
            excl_new = (incl > excl) ? incl : excl;
 
            /* current max including i */
            incl = excl + arr[i];
            excl = excl_new;
        }
 
        /* return max of incl and excl */
        return ((incl > excl) ? incl : excl);
    }
 
    // Driver program to test above functions
    public static void main(String[] args)
    {
        MaximumSum sum = new MaximumSum();
        int arr[] = new int[]{5, 5, 10, 100, 10, 5};
        System.out.println(sum.FindMaxSum(arr, arr.length));
    }
}
 
// This code has been contributed by Mayank Jaiswal

Python

# Function to return max sum such that
# no two elements are adjacent
def find_max_sum(arr):
    incl = 0
    excl = 0
    
    for i in arr:
         
        # Current max excluding i (No ternary in
        # Python)
        new_excl = excl if excl>incl else incl
        
        # Current max including i
        incl = excl + i
        excl = new_excl
     
    # return max of incl and excl
    return (excl if excl>incl else incl)
 
# Driver program to test above function
arr = [5, 5, 10, 100, 10, 5]
print find_max_sum(arr)
 
# This code is contributed by Kalai Selvan

C#

/* Program to return max sum such that no
two elements are adjacent */
using System;
 
class GFG {
     
    /* Function to return max sum such
    that no two elements are adjacent */
    static int FindMaxSum(int []arr, int n)
    {
        int incl = arr[0];
        int excl = 0;
        int excl_new;
        int i;
 
        for (i = 1; i < n; i++)
        {
            /* current max excluding i */
            excl_new = (incl > excl) ?
                            incl : excl;
 
            /* current max including i */
            incl = excl + arr[i];
            excl = excl_new;
        }
 
        /* return max of incl and excl */
        return ((incl > excl) ?
                            incl : excl);
    }
 
    // Driver program to test above
    // functions
    public static void Main()
    {
        int []arr = new int[]{5, 5, 10,
                              100, 10, 5};
                               
        Console.Write(
             FindMaxSum(arr, arr.Length));
    }
}
 
// This code has been contributed by
// nitin mittal

PHP

<?php
// PHP code to find Maximum sum
// such that no two elements
// are adjacent
 
/* Function to return max sum
   such that no two elements
   are adjacent */
function FindMaxSum($arr, $n)
{
    $incl = $arr[0];
    $excl = 0;
    $excl_new;
    $i;
 
for ($i = 1; $i <$n; $i++)
{
     
    // current max excluding i
    $excl_new = ($incl > $excl)? $incl: $excl;
 
    // current max including i
    $incl = $excl + $arr[$i];
    $excl = $excl_new;
}
 
// return max of incl and excl
return (($incl > $excl)? $incl : $excl);
}
 
// Driver Code
$arr = array(5, 5, 10, 100, 10, 5);
$n = sizeof($arr);
echo FindMaxSum($arr, $n);
     
// This code is contributed by Ajit
?>

Javascript

<script>
 
// Javascript program for the above approach
 
// Function to return max sum such that
// no two elements are adjacent
function FindMaxSum(arr, n)
{
    let incl = arr[0];
    let excl = 0;
    let excl_new;
    let i;
 
    for(i = 1; i < n; i++)
    {
         
        // Current max excluding i
        excl_new = (incl > excl) ? incl : excl;
 
        // Current max including i
        incl = excl + arr[i];
        excl = excl_new;
    }
 
    // Return max of incl and excl
    return ((incl > excl) ? incl : excl);
}
 
// Driver code
let arr = [ 5, 5, 10, 100, 10, 5 ];
 
document.write(FindMaxSum(arr, arr.length));
 
// This code is contributed by Mayank Tyagi
 
</script>

Output:

Time Complexity: O(n)

Refer Find maximum possible stolen value from houses for more explanation.
Now try the same problem for an array with negative numbers also.
Please write comments if you find any bug in the above program/algorithm or other ways to solve the same problem.

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