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# Count of all subsequences having adjacent elements with different parity

• Last Updated : 11 May, 2021

Given an array arr[] of size N, the task is to find the number of non-empty subsequences from the given array such that no two adjacent elements of the subsequence have the same parity.

Examples:

Input: arr[] = [5, 6, 9, 7]
Output:
Explanation:
All such subsequences of given array will be {5}, {6}, {9}, {7}, {5, 6}, {6, 7}, {6, 9}, {5, 6, 9}, {5, 6, 7}.
Input: arr[] = [2, 3, 4, 8]
Output: 9

Naive Approach: Generate all non-empty subsequences and select the ones with alternate odd-even or even-odd numbers and count all such subsequences to obtain the answer.
Time Complexity: O(2N)
Efficient Approach:
The above approach can be optimized using Dynamic Programming. Follow the steps below to solve the problem:

• Consider a dp[] matrix of dimensions (N+1)*(2).
• dp[i] stores the count of subsequences till ith index ending with an even element.
• dp[i] stores the count of subsequences till ith index ending with an odd element.
• Hence, for every ith element, check if the element is even or odd and proceed by including as well as excluding the ith element.
• Hence, the recurrence relation if the ith element is odd:

dp[i] = dp[i – 1] (Including the ith element by considering all subsequences ending with even element till (i – 1)th index) + 1 + dp[i – 1] (Excluding the ith element)

• Similarly, if the ith element is even:

dp[i] = dp[i – 1] (Including the ith element by considering all subsequences ending with odd element till (i – 1)th index) + 1 + dp[i – 1] (Excluding the ith element)

• Finally, the sum of dp[n], which contains all such subsequences ending with an even element, and dp[n], which contains all such subsequences ending with an odd element, is the required answer.

Below is the implementation of the above approach:

## C++

 `// C++ Program to implement the``// above approach``#include ``using` `namespace` `std;` `// Function to find required subsequences``int` `validsubsequences(``int` `arr[], ``int` `n)``{``    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with even element``    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with odd element``    ``long` `long` `int` `dp[n + 1];` `    ``// Initialise the dp[][] with 0.``    ``for` `(``int` `i = 0; i < n + 1; i++) {``        ``dp[i] = 0;``        ``dp[i] = 0;``    ``}` `    ``for` `(``int` `i = 1; i <= n; i++) {` `        ``// If odd element is``        ``// encountered``        ``if` `(arr[i - 1] % 2) {` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i] += 1;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with even element``            ``// till (i-1)th indexes``            ``dp[i] += dp[i - 1];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i] += dp[i - 1];` `            ``dp[i] += dp[i - 1];``        ``}``        ``else` `{` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i] += 1;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with odd element``            ``// till (i-1)th indexes``            ``dp[i] += dp[i - 1];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i] += dp[i - 1];``            ``dp[i] += dp[i - 1];``        ``}``    ``}` `    ``// Count of all valid subsequences``    ``return` `dp[n] + dp[n];``}` `// Driver Code``int` `main()``{``    ``int` `arr[] = { 5, 6, 9, 7 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``cout << validsubsequences(arr, n);` `    ``return` `0;``}`

## Java

 `// Java Program implementation``// of the approach``import` `java.util.*;``import` `java.io.*;` `class` `GFG{` `// Function to find required subsequences    ``static` `int` `validsubsequences(``int` `arr[], ``int` `n)``{``    ` `    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with even element``    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with odd element``    ``long` `dp[][] = ``new` `long` `[n + ``1``][``2``];` `    ``// Initialise the dp[][] with 0.``    ``for``(``int` `i = ``0``; i < n + ``1``; i++)``    ``{``        ``dp[i][``0``] = ``0``;``        ``dp[i][``1``] = ``0``;``    ``}``    ` `    ``for``(``int` `i = ``1``; i <= n; i++)``    ``{``        ` `        ``// If odd element is``        ``// encountered``        ``if` `(arr[i - ``1``] % ``2` `!= ``0``)``        ``{` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i][``1``] += ``1``;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with even element``            ``// till (i-1)th indexes``            ``dp[i][``1``] += dp[i - ``1``][``0``];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i][``1``] += dp[i - ``1``][``1``];``            ``dp[i][``0``] += dp[i - ``1``][``0``];``        ``}``        ``else``        ``{` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i][``0``] += ``1``;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with odd element``            ``// till (i-1)th indexes``            ``dp[i][``0``] += dp[i - ``1``][``1``];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i][``0``] += dp[i - ``1``][``0``];``            ``dp[i][``1``] += dp[i - ``1``][``1``];``        ``}``    ``}` `    ``// Count of all valid subsequences``    ``return` `(``int``)(dp[n][``0``] + dp[n][``1``]);``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``5``, ``6``, ``9``, ``7` `};``    ``int` `n = arr.length;``        ` `    ``System.out.print(validsubsequences(arr, n));``}``}` `// This code is contributed by code_hunt`

## Python3

 `# Python3 program to implement the``# above approach` `# Function to find required subsequences``def` `validsubsequences(arr, n):` `    ``# dp[i]: Stores the number of``    ``# subsequences till i-th index``    ``# ending with even element``    ``# dp[i]: Stores the number of``    ``# subsequences till i-th index``    ``# ending with odd element` `    ``# Initialise the dp[][] with 0.``    ``dp ``=` `[[``0` `for` `i ``in` `range``(``2``)]``             ``for` `j ``in` `range``(n ``+` `1``)]``             ` `    ``for` `i ``in` `range``(``1``, n ``+` `1``):` `        ``# If odd element is``        ``# encountered``        ``if``(arr[i ``-` `1``] ``%` `2``):` `            ``# Considering i-th element``            ``# will be present in``            ``# the subsequence``            ``dp[i][``1``] ``+``=` `1` `            ``# Appending i-th element to all``            ``# non-empty subsequences``            ``# ending with even element``            ``# till (i-1)th indexes``            ``dp[i][``1``] ``+``=` `dp[i ``-` `1``][``0``]` `            ``# Considering ith element will``            ``# not be present in``            ``# the subsequence``            ``dp[i][``1``] ``+``=` `dp[i ``-` `1``][``1``]``            ``dp[i][``0``] ``+``=` `dp[i ``-` `1``][``0``]` `        ``else``:` `            ``# Considering i-th element``            ``# will be present in``            ``# the subsequence``            ``dp[i][``0``] ``+``=` `1` `            ``# Appending i-th element to all``            ``# non-empty subsequences``            ``# ending with odd element``            ``# till (i-1)th indexes``            ``dp[i][``0``] ``+``=` `dp[i ``-` `1``][``1``]` `            ``# Considering ith element will``            ``# not be present in``            ``# the subsequence``            ``dp[i][``0``] ``+``=` `dp[i ``-` `1``][``0``]``            ``dp[i][``1``] ``+``=` `dp[i ``-` `1``][``1``]` `    ``# Count of all valid subsequences``    ``return` `dp[n][``0``] ``+` `dp[n][``1``]` `# Driver code``if` `__name__ ``=``=` `'__main__'``:` `    ``arr ``=` `[ ``5``, ``6``, ``9``, ``7` `]``    ``n ``=` `len``(arr)` `    ``print``(validsubsequences(arr, n))` `# This code is contributed by Shivam Singh`

## C#

 `// C# program implementation``// of the approach``using` `System;` `class` `GFG{` `// Function to find required subsequences    ``static` `int` `validsubsequences(``int``[] arr, ``int` `n)``{``    ` `    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with even element``    ``// dp[i]: Stores the number of``    ``// subsequences till i-th index``    ``// ending with odd element``    ``long``[,] dp = ``new` `long` `[n + 1, 2];` `    ``// Initialise the dp[][] with 0.``    ``for``(``int` `i = 0; i < n + 1; i++)``    ``{``        ``dp[i, 0] = 0;``        ``dp[i, 1] = 0;``    ``}``    ` `    ``for``(``int` `i = 1; i <= n; i++)``    ``{``        ` `        ``// If odd element is``        ``// encountered``        ``if` `(arr[i - 1] % 2 != 0)``        ``{` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i, 1] += 1;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with even element``            ``// till (i-1)th indexes``            ``dp[i, 1] += dp[i - 1, 0];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i, 1] += dp[i - 1, 1];``            ``dp[i, 0] += dp[i - 1, 0];``        ``}``        ``else``        ``{` `            ``// Considering i-th element``            ``// will be present in``            ``// the subsequence``            ``dp[i, 0] += 1;` `            ``// Appending i-th element to all``            ``// non-empty subsequences``            ``// ending with odd element``            ``// till (i-1)th indexes``            ``dp[i, 0] += dp[i - 1, 1];` `            ``// Considering ith element will``            ``// not be present in``            ``// the subsequence``            ``dp[i, 0] += dp[i - 1, 0];``            ``dp[i, 1] += dp[i - 1, 1];``        ``}``    ``}` `    ``// Count of all valid subsequences``    ``return` `(``int``)(dp[n, 0] + dp[n, 1]);``}` `// Driver code``public` `static` `void` `Main()``{``    ``int``[] arr = { 5, 6, 9, 7 };``    ``int` `n = arr.Length;``        ` `    ``Console.Write(validsubsequences(arr, n));``}``}` `// This code is contributed by chitranayal`

## Javascript

 ``
Output:
`9`

Time complexity: O(N)
Auxiliary Space complexity: O(N)

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