# Largest divisible pairs subset

• Difficulty Level : Medium
• Last Updated : 21 Apr, 2021

Given an array of n distinct elements, find length of the largest subset such that every pair in the subset is such that the larger element of the pair is divisible by smaller element.

Examples:

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```Input : arr[] = {10, 5, 3, 15, 20}
Output : 3
Explanation: The largest subset is 10, 5, 20.
10 is divisible by 5, and 20 is divisible by 10.

Input : arr[] = {18, 1, 3, 6, 13, 17}
Output : 4
Explanation: The largest subset is 18, 1, 3, 6,
In the subsequence, 3 is divisible by 1,
6 by 3 and 18 by 6.```

This can be solved using Dynamic Programming. We traverse the sorted array from the end. For every element a[i], we compute dp[i] where dp[i] indicates size of largest divisible subset where a[i] is the smallest element. We can compute dp[i] in array using values from dp[i+1] to dp[n-1]. Finally, we return the maximum value from dp[].

Below is the implementation of the above approach:

## C++

 `// CPP program to find the largest subset which``// where each pair is divisible.``#include ``using` `namespace` `std;` `// function to find the longest Subsequence``int` `largestSubset(``int` `a[], ``int` `n)``{``    ``// dp[i] is going to store size of largest``    ``// divisible subset beginning with a[i].``    ``int` `dp[n];` `    ``// Since last element is largest, d[n-1] is 1``    ``dp[n - 1] = 1;` `    ``// Fill values for smaller elements.``    ``for` `(``int` `i = n - 2; i >= 0; i--) {` `        ``// Find all multiples of a[i] and consider``        ``// the multiple that has largest subset``        ``// beginning with it.``        ``int` `mxm = 0;``        ``for` `(``int` `j = i + 1; j < n; j++)``            ``if` `(a[j] % a[i] == 0 || a[i] % a[j] == 0)``                ``mxm = max(mxm, dp[j]);` `        ``dp[i] = 1 + mxm;``    ``}` `    ``// Return maximum value from dp[]``    ``return` `*max_element(dp, dp + n);``}` `// driver code to check the above function``int` `main()``{``    ``int` `a[] = { 1, 3, 6, 13, 17, 18 };``    ``int` `n = ``sizeof``(a) / ``sizeof``(a);``    ``cout << largestSubset(a, n) << endl;``    ``return` `0;``}`

## Java

 `import` `java.util.Arrays;` `// Java program to find the largest``// subset which was each pair``// is divisible.``class` `GFG {` `    ``// function to find the longest Subsequence``    ``static` `int` `largestSubset(``int``[] a, ``int` `n)``    ``{``        ``// dp[i] is going to store size of largest``        ``// divisible subset beginning with a[i].``        ``int``[] dp = ``new` `int``[n];` `        ``// Since last element is largest, d[n-1] is 1``        ``dp[n - ``1``] = ``1``;` `        ``// Fill values for smaller elements.``        ``for` `(``int` `i = n - ``2``; i >= ``0``; i--) {` `            ``// Find all multiples of a[i] and consider``            ``// the multiple that has largest subset``            ``// beginning with it.``            ``int` `mxm = ``0``;``            ``for` `(``int` `j = i + ``1``; j < n; j++) {``                ``if` `(a[j] % a[i] == ``0` `|| a[i] % a[j] == ``0``) {``                    ``mxm = Math.max(mxm, dp[j]);``                ``}``            ``}` `            ``dp[i] = ``1` `+ mxm;``        ``}` `        ``// Return maximum value from dp[]``        ``return` `Arrays.stream(dp).max().getAsInt();``    ``}` `    ``// driver code to check the above function``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int``[] a = { ``1``, ``3``, ``6``, ``13``, ``17``, ``18` `};``        ``int` `n = a.length;``        ``System.out.println(largestSubset(a, n));``    ``}``}` `/* This JAVA code is contributed by Rajput-Ji*/`

## Python3

 `# Python program to find the largest``# subset where each pair is divisible.` `# function to find the longest Subsequence``def` `largestSubset(a, n):``    ` `    ``# dp[i] is going to store size``    ``# of largest divisible subset``    ``# beginning with a[i].``    ``dp ``=` `[``0` `for` `i ``in` `range``(n)]``    ` `    ``# Since last element is largest,``    ``# d[n-1] is 1``    ``dp[n ``-` `1``] ``=` `1``;` `    ``# Fill values for smaller elements``    ``for` `i ``in` `range``(n ``-` `2``, ``-``1``, ``-``1``):``        ` `        ``# Find all multiples of a[i]``        ``# and consider the multiple``        ``# that has largest subset    ``        ``# beginning with it.``        ``mxm ``=` `0``;``        ``for` `j ``in` `range``(i ``+` `1``, n):``            ``if` `a[j] ``%` `a[i] ``=``=` `0` `or` `a[i] ``%` `a[j] ``=``=` `0``:``                ``mxm ``=` `max``(mxm, dp[j])``        ``dp[i] ``=` `1` `+` `mxm``        ` `    ``# Return maximum value from dp[]``    ``return` `max``(dp)` `# Driver Code``a ``=` `[ ``1``, ``3``, ``6``, ``13``, ``17``, ``18` `]``n ``=` `len``(a)``print``(largestSubset(a, n))` `# This code is contributed by``# sahil shelangia`

## C#

 `// C# program to find the largest``// subset which where each pair``// is divisible.``using` `System;``using` `System.Linq;` `public` `class` `GFG {` `    ``// function to find the longest Subsequence``    ``static` `int` `largestSubset(``int``[] a, ``int` `n)``    ``{``        ``// dp[i] is going to store size of largest``        ``// divisible subset beginning with a[i].``        ``int``[] dp = ``new` `int``[n];` `        ``// Since last element is largest, d[n-1] is 1``        ``dp[n - 1] = 1;` `        ``// Fill values for smaller elements.``        ``for` `(``int` `i = n - 2; i >= 0; i--) {` `            ``// Find all multiples of a[i] and consider``            ``// the multiple that has largest subset``            ``// beginning with it.``            ``int` `mxm = 0;``            ``for` `(``int` `j = i + 1; j < n; j++)``                ``if` `(a[j] % a[i] == 0 | a[i] % a[j] == 0)``                    ``mxm = Math.Max(mxm, dp[j]);` `            ``dp[i] = 1 + mxm;``        ``}` `        ``// Return maximum value from dp[]``        ``return` `dp.Max();``    ``}` `    ``// driver code to check the above function``    ``static` `public` `void` `Main()``    ``{``        ``int``[] a = { 1, 3, 6, 13, 17, 18 };``        ``int` `n = a.Length;``        ``Console.WriteLine(largestSubset(a, n));``    ``}``}` `// This code is contributed by vt_m.`

## PHP

 `= 0; ``\$i``--)``    ``{` `        ``// Find all multiples of``        ``// a[i] and consider``        ``// the multiple that``        ``// has largest subset``        ``// beginning with it.``        ``\$mxm` `= 0;``        ``for` `(``\$j` `= ``\$i` `+ 1; ``\$j` `< ``\$n``; ``\$j``++)``            ``if` `(``\$a``[``\$j``] % ``\$a``[``\$i``] == 0 ``or` `\$a``[``\$i``] % ``\$a``[``\$j``] == 0)``                ``\$mxm` `= max(``\$mxm``, ``\$dp``[``\$j``]);` `        ``\$dp``[``\$i``] = 1 + ``\$mxm``;``    ``}` `    ``// Return maximum value``    ``// from dp[]``    ``return` `max(``\$dp``);``}` `    ``// Driver Code``    ``\$a` `= ``array``(1, 3, 6, 13, 17, 18);``    ``\$n` `= ``count``(``\$a``);``    ``echo` `largestSubset(``\$a``, ``\$n``);``    ` `// This code is contributed by anuj_67.``?>`

## Javascript

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Output:

`4`

Time Complexity: O(n*n)

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