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Maximum sum such that no two elements are adjacent | Set 2

  • Difficulty Level : Medium
  • Last Updated : 24 Nov, 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).

Examples: 

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Input :  arr[] = {3, 5, 3} 
Output : 6 
Explanation : 
Selecting indexes 0 and 2 will maximise the sum 
i.e 3+3 = 6

Input : arr[] = {2, 5, 2}
Output : 5 

We have already discussed the efficient approach of solving this problem in the previous article.
However, we can also solve this problem using the Dynamic Programming approach.
Dynamic Programming Approach: Let’s decide the states of ‘dp’. Let dp[i] be the largest possible sum for the sub-array starting from index ‘i’ and ending at index ‘N-1’. Now, we have to find a recurrence relation between this state and a lower-order state.
In this case for an index ‘i’, we will have two choices.  



1) Choose the current index:
   In this case, the relation will be dp[i] = arr[i] + dp[i+2]
2) Skip the current index:
   Relation will be dp[i] = dp[i+1]

We will choose the path that maximizes our result. 
Thus, the final relation will be:  

dp[i] = max(dp[i+2]+arr[i], dp[i+1])

Below is the implementation of the above approach:  

C++




// C++ program to implement above approach
 
#include <bits/stdc++.h>
#define maxLen 10
using namespace std;
 
// variable to store states of dp
int dp[maxLen];
 
// variable to check if a given state
// has been solved
bool v[maxLen];
 
// Function to find the maximum sum subsequence
// such that no two elements are adjacent
int maxSum(int arr[], int i, int n)
{
    // Base case
    if (i >= n)
        return 0;
 
    // To check if a state has
    // been solved
    if (v[i])
        return dp[i];
    v[i] = 1;
 
    // Required recurrence relation
    dp[i] = max(maxSum(arr, i + 1, n),
                arr[i] + maxSum(arr, i + 2, n));
 
    // Returning the value
    return dp[i];
}
 
// Driver code
int main()
{
    int arr[] = { 12, 9, 7, 33 };
 
    int n = sizeof(arr) / sizeof(int);
 
    cout << maxSum(arr, 0, n);
 
    return 0;
}

Java




// Java program to implement above approach
class GFG
{
 
static int maxLen = 10;
 
// variable to store states of dp
static int dp[] = new int[maxLen];
 
// variable to check if a given state
// has been solved
static boolean v[] = new boolean[maxLen];
 
// Function to find the maximum sum subsequence
// such that no two elements are adjacent
static int maxSum(int arr[], int i, int n)
{
    // Base case
    if (i >= n)
        return 0;
 
    // To check if a state has
    // been solved
    if (v[i])
        return dp[i];
    v[i] = true;
 
    // Required recurrence relation
    dp[i] = Math.max(maxSum(arr, i + 1, n),
                arr[i] + maxSum(arr, i + 2, n));
 
    // Returning the value
    return dp[i];
}
 
// Driver code
public static void main(String args[])
{
    int arr[] = { 12, 9, 7, 33 };
    int n = arr.length;
    System.out.println( maxSum(arr, 0, n));
}
}
 
// This code is contributed by Arnab Kundu

Python3




# Python 3 program to implement above approach
maxLen = 10
 
# variable to store states of dp
dp = [0 for i in range(maxLen)]
 
# variable to check if a given state
# has been solved
v = [0 for i in range(maxLen)]
 
# Function to find the maximum sum subsequence
# such that no two elements are adjacent
def maxSum(arr, i, n):
    # Base case
    if (i >= n):
        return 0
 
    # To check if a state has
    # been solved
    if (v[i]):
        return dp[i]
    v[i] = 1
 
    # Required recurrence relation
    dp[i] = max(maxSum(arr, i + 1, n),
            arr[i] + maxSum(arr, i + 2, n))
 
    # Returning the value
    return dp[i]
 
# Driver code
if __name__ == '__main__':
    arr = [12, 9, 7, 33]
 
    n = len(arr)
    print(maxSum(arr, 0, n))
 
# This code is contributed by
# Surendra_Gangwar

C#




// C# program to implement above approach
using System;
 
class GFG
{
 
static int maxLen = 10;
 
// variable to store states of dp
static int[] dp = new int[maxLen];
 
// variable to check if a given state
// has been solved
static bool[] v = new bool[maxLen];
 
// Function to find the maximum sum subsequence
// such that no two elements are adjacent
static int maxSum(int[] arr, int i, int n)
{
    // Base case
    if (i >= n)
        return 0;
 
    // To check if a state has
    // been solved
    if (v[i])
        return dp[i];
    v[i] = true;
 
    // Required recurrence relation
    dp[i] = Math.Max(maxSum(arr, i + 1, n),
                arr[i] + maxSum(arr, i + 2, n));
 
    // Returning the value
    return dp[i];
}
 
// Driver code
public static void Main()
{
    int[] arr = { 12, 9, 7, 33 };
    int n = arr.Length;
    Console.Write( maxSum(arr, 0, n));
}
}
 
// This code is contributed by ChitraNayal

PHP




<?php
// PHP program to implement above approach
 
$maxLen = 10;
 
// variable to store states of dp
$dp = array_fill(0, $GLOBALS['maxLen'], 0);
 
// variable to check if a given state
// has been solved
$v = array_fill(0, $GLOBALS['maxLen'], 0);
 
// Function to find the maximum sum subsequence
// such that no two elements are adjacent
function maxSum($arr, $i, $n)
{
    // Base case
    if ($i >= $n)
        return 0;
 
    // To check if a state has
    // been solved
    if ($GLOBALS['v'][$i])
        return $GLOBALS['dp'][$i];
         
    $GLOBALS['v'][$i] = 1;
 
    // Required recurrence relation
    $GLOBALS['dp'][$i] = max(maxSum($arr, $i + 1, $n),
                $arr[$i] + maxSum($arr, $i + 2, $n));
 
    // Returning the value
    return $GLOBALS['dp'][$i];
}
 
    // Driver code
    $arr = array( 12, 9, 7, 33 );
 
    $n = count($arr);
 
    echo maxSum($arr, 0, $n);
 
    // This code is contributed by AnkitRai01
?>

Javascript




<script>
 
// Javascript program to implement above approach
var maxLen = 10;
 
// variable to store states of dp
var dp = Array(maxLen);
 
// variable to check if a given state
// has been solved
var v = Array(maxLen);
 
// Function to find the maximum sum subsequence
// such that no two elements are adjacent
function maxSum(arr, i, n)
{
    // Base case
    if (i >= n)
        return 0;
 
    // To check if a state has
    // been solved
    if (v[i])
        return dp[i];
    v[i] = 1;
 
    // Required recurrence relation
    dp[i] = Math.max(maxSum(arr, i + 1, n),
                arr[i] + maxSum(arr, i + 2, n));
 
    // Returning the value
    return dp[i];
}
 
// Driver code
var arr = [12, 9, 7, 33 ];
var n = arr.length;
document.write( maxSum(arr, 0, n));
 
</script>
Output: 
45

 




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