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# Minimum length of subsequence having unit GCD

Given an array arr[] of N positive integers. The task is to find the length of the shortest sub-sequence such that the GCD of the subsequence is 1. If none of the sub-sequence has GCD 1, then print “-1“.

Examples:

Input: arr[] = {2, 6, 12, 3}
Output: 2
Explanation:
The GCD of 2, 3 = 1, which is the smallest length of subsequence like 2.

Input: arr[] = {2, 4}
Output: -1
Explanation:
GCD of 2, 4 = 2

Naive Approach: The idea is to generate all possible subsequences of the given array and print the length of that subsequence whose GCD is unity and has a minimum length. If none of the sub-sequences has GCD 1, then print “-1“.

Time Complexity: O(2N
Auxiliary Space: O(1)

Another Approach: Using the Dynamic programming Tabulation method

We use a 2D matrix to store the previous computation of sub-problems and find the actual answer.

Implementation steps :

• Create a table to store the solution of the subproblems.
• Initialize the table with base cases
• Fill up the table iteratively
• Return the final solution

Implementation:

## C++

 // C++ program for the above approach#include using namespace std; int findMinimumLength(vector& a){    int n = a.size();    vector > dp(n, vector(n, 0));     // Calculate the GCD of all elements in the sequence    int gcd = a[0];    for (int i = 1; i < n; i++) {        gcd = __gcd(gcd, a[i]);    }     // If the GCD is not 1, it's impossible to form a    // subsequence with GCD 1    if (gcd != 1) {        return -1;    }     // Initialize the diagonal of the dp table to 1    for (int i = 0; i < n; i++) {        dp[i][i] = 1;    }     // Calculate the dp table    for (int len = 2; len <= n; len++) {        for (int i = 0; i < n - len + 1; i++) {            int j = i + len - 1;            int g = __gcd(a[i], a[j]);            if (g == 1) {                dp[i][j] = 2;            }            else {                dp[i][j] = INT_MAX;                for (int k = i; k < j; k++) {                    dp[i][j] = min(dp[i][j],                                   dp[i][k] + dp[k + 1][j]);                }            }        }    }     return dp[0][n - 1];} // Driver Codeint main(){    vector a = { 2, 6, 12, 3 };    // function call    int ans = findMinimumLength(a);    if (ans == -1) {        cout << -1 << endl;    }    else {        cout << ans << endl;    }    return 0;} // this code is contributed by bhardwajji

## Java

 import java.util.*; public class Main {   public static int    findMinimumLength(ArrayList a)  {    int n = a.size();    int[][] dp = new int[n][n];     // Calculate the GCD of all elements in the sequence    int gcd = a.get(0);    for (int i = 1; i < n; i++) {      gcd = gcd(gcd, a.get(i));    }     // If the GCD is not 1, it's impossible to form a    // subsequence with GCD 1    if (gcd != 1) {      return -1;    }     // Initialize the diagonal of the dp table to 1    for (int i = 0; i < n; i++) {      dp[i][i] = 1;    }     // Calculate the dp table    for (int len = 2; len <= n; len++) {      for (int i = 0; i < n - len + 1; i++) {        int j = i + len - 1;        int g = gcd(a.get(i), a.get(j));        if (g == 1) {          dp[i][j] = 2;        }        else {          dp[i][j] = Integer.MAX_VALUE;          for (int k = i; k < j; k++) {            dp[i][j] = Math.min(              dp[i][j],              dp[i][k] + dp[k + 1][j]);          }        }      }    }     return dp[0][n - 1];  }   public static int gcd(int a, int b)  {    if (b == 0) {      return a;    }    else {      return gcd(b, a % b);    }  }   public static void main(String[] args)  {    ArrayList a      = new ArrayList<>(Arrays.asList(2, 6, 12, 3));         // function call    int ans = findMinimumLength(a);    if (ans == -1) {      System.out.println(-1);    }    else {      System.out.println(ans);    }  }} // This code is contributed by user_dtewbxkn77n

## Python3

 # Python program for the above approach import math  def findMinimumLength(a):    n = len(a)    dp = [[0] * n for _ in range(n)]     # Calculate the GCD of all elements in the sequence    gcd = a[0]    for i in range(1, n):        gcd = math.gcd(gcd, a[i])     # If the GCD is not 1, it's impossible to form a subsequence with GCD 1    if gcd != 1:        return -1     # Initialize the diagonal of the dp table to 1    for i in range(n):        dp[i][i] = 1     # Calculate the dp table    for length in range(2, n+1):        for i in range(n - length + 1):            j = i + length - 1            g = math.gcd(a[i], a[j])            if g == 1:                dp[i][j] = 2            else:                dp[i][j] = float('inf')                for k in range(i, j):                    dp[i][j] = min(dp[i][j], dp[i][k] + dp[k+1][j])     return dp[0][n-1]  # Driver Codea = [2, 6, 12, 3]ans = findMinimumLength(a)if ans == -1:    print(-1)else:    print(ans) # Contributed by adityasha4x71

## Javascript

 function findMinimumLength(a) {    const n = a.length;    const dp = Array.from({        length: n    }, () => new Array(n).fill(0));     // Calculate the GCD of all elements in the sequence    let gcd = a[0];    for (let i = 1; i < n; i++) {        gcd = gcdFunc(gcd, a[i]);    }     // If the GCD is not 1, it's impossible to form a subsequence with GCD 1    if (gcd !== 1) {        return -1;    }     // Initialize the diagonal of the dp table to 1    for (let i = 0; i < n; i++) {        dp[i][i] = 1;    }     // Calculate the dp table    for (let len = 2; len <= n; len++) {        for (let i = 0; i < n - len + 1; i++) {            const j = i + len - 1;            const g = gcdFunc(a[i], a[j]);            if (g === 1) {                dp[i][j] = 2;            } else {                dp[i][j] = Infinity;                for (let k = i; k < j; k++) {                    dp[i][j] = Math.min(dp[i][j], dp[i][k] + dp[k + 1][j]);                }            }        }    }     return dp[0][n - 1];} function gcdFunc(a, b) {    if (b === 0) {        return a;    } else {        return gcdFunc(b, a % b);    }} const a = [2, 6, 12, 3];const ans = findMinimumLength(a);if (ans === -1) {    console.log(-1);} else {    console.log(ans);}

## C#

 using System;using System.Collections.Generic;using System.Linq; class Program {    static int FindMinimumLength(List a)    {        int n = a.Count;        int[, ] dp = new int[n, n];         // Calculate the GCD of all elements in the sequence        int gcd = a[0];        for (int i = 1; i < n; i++) {            gcd = Gcd(gcd, a[i]);        }         // If the GCD is not 1, it's impossible to form a        // subsequence with GCD 1        if (gcd != 1) {            return -1;        }         // Initialize the diagonal of the dp table to 1        for (int i = 0; i < n; i++) {            dp[i, i] = 1;        }         // Calculate the dp table        for (int len = 2; len <= n; len++) {            for (int i = 0; i < n - len + 1; i++) {                int j = i + len - 1;                int g = Gcd(a[i], a[j]);                if (g == 1) {                    dp[i, j] = 2;                }                else {                    dp[i, j] = int.MaxValue;                    for (int k = i; k < j; k++) {                        dp[i, j] = Math.Min(                            dp[i, j],                            dp[i, k] + dp[k + 1, j]);                    }                }            }        }         return dp[0, n - 1];    }     static int Gcd(int a, int b)    {        if (a == 0) {            return b;        }        return Gcd(b % a, a);    }     static void Main(string[] args)    {        List a = new List() { 2, 6, 12, 3 };        // function call        int ans = FindMinimumLength(a);        if (ans == -1) {            Console.WriteLine(-1);        }        else {            Console.WriteLine(ans);        }    }}

Output

2

Time Complexity: O(n^3)
Auxiliary Space: O(n^2)

Efficient Approach: There are 2 key observations for solving this problem:

1. Two numbers will have their GCD equal to one only when their prime factors are different.
2. Any positive number which is less than 109 can have a maximum of 9 prime factors.
For Example 2×3×5×7×11×13×17×19×23 = 22, 30, 92, 870. If we multiply this number by the next prime number, which is 29, it will be greater than 10^9.

Follow the steps below to solve the problem:

1. Express the numbers as the product of its prime factors. Since we have a maximum of 9 prime factors, we can use the concept of Bitmask to store the state of the number.
For Example, prime factors of 12 are 2 and 3. This can be expressed in binary as 11 (Ignoring the preceding zeroes), meaning two prime factors are there for this number.
2. For every number in the input array, check if any other number has the corresponding bit set or not. This could be achieved using Bitwise AND operation. The resultant of this operation is another state of our solution space.
3. Now use the concept of Dynamic Programming to memorize the states. The idea is to use an array to store the states of the solution space. This works since only 9 bits could be set a time, and an array of size 1024 could capture all the states of the solution space.
4. Every state uses dynamic programming to store the shortest way to reach that state.
5. If the Bitwise AND of any two states is equal to 0, then the GCD is equal to one, i.e., if there is a possibility to reach state 0 from the current state, then it will have the minimum length sub-sequence and print that length otherwise print “-1”.

Below is the implementation of the above approach:

## C++

 // C++ program for the above approach#include using namespace std; // Function that finds the prime// factors of a numbervector findPrimeFactors(int n){    // To store the prime factor    vector primeFactors(9, 0);     int j = 0;     // 2s that divide n    if (n % 2 == 0) {        primeFactors[j++] = 2;        while (n % 2 == 0)            n >>= 1;    }     // N must be odd at this point    // Skip one element    for (int i = 3;         i * i <= n; i += 2) {         if (n % i == 0) {             // Update the prime factor            primeFactors[j++] = i;            while (n % i == 0)                n /= i;        }    }     // If n is a prime number    // greater than 2    if (n > 2)        primeFactors[j++] = n;         vector PrimeFactors(j);         for(int i = 0; i < j; i++)    {        PrimeFactors[i] = primeFactors[i];    }         return PrimeFactors;} // Function that finds the shortest// subsequencevoid findShortestSubsequence(vector &dp, vector a,                        int index, vector primeFactors){    int n = a.size();     for (int j = index; j < n; j++) {        int bitmask = 0;         for (int p = 0;             p < primeFactors.size(); p++) {             // Check if the prime factor            // of first number, is also            // the prime factor of the            // rest numbers in array            if ((a[j] % primeFactors[p]) == 0) {                 // Set corresponding bit                // of prime factor to 1,                // it means both these                // numbers have the                // same prime factor                bitmask ^= (1 << p);            }        }         for (int i = 0; i < dp.size(); i++) {             // If no states encountered            // so far continue for this            // combination of bits            if (dp[i] == n + 1)                continue;             // Update this state with            // minimum ways to reach            // this state            dp[bitmask & i]                = min(dp[bitmask & i],                           dp[i] + 1);        }    }} // Function that print the minimum// length of subsequencevoid printMinimumLength(vector a){    int Min = a.size() + 1;     for (int i = 0; i < a.size() - 1; i++) {         // Find the prime factors of        // the first number        vector primeFactors            = findPrimeFactors(a[i]);         int n = primeFactors.size();             // Initialize the array with        // maximum steps, size of the        // array + 1 for instance        vector dp(1 << n, a.size() + 1);         // Express the prime factors        // in bit representation         // Total number of set bits is        // equal to the total number        // of prime factors        int setBits = (1 << n) - 1;         // Indicates there is one        // way to reach the number        // under consideration        dp[setBits] = 1;        findShortestSubsequence(dp, a, i + 1,                                primeFactors);         // State 0 corresponds        // to gcd of 1        Min = min(dp[0], Min);    }     // If not found such subsequence    // then print "-1"    if (Min == (a.size() + 1))        cout << -1 << endl;     // Else print the length    else        cout << Min << endl;} // Driver codeint main(){    // Given array arr[]    vector arr = { 2, 6, 12, 3 };         // Function Call    printMinimumLength(arr);    return 0;} // This code is contributed by divyeshrabadiya07

## Java

 // Java program for the above approachimport java.io.*;import java.util.*; class GFG {     // Function that finds the prime    // factors of a number    private static int[] findPrimeFactors(int n)    {        // To store the prime factor        int[] primeFactors = new int[9];         int j = 0;         // 2s that divide n        if (n % 2 == 0) {            primeFactors[j++] = 2;            while (n % 2 == 0)                n >>= 1;        }         // N must be odd at this point        // Skip one element        for (int i = 3;             i * i <= n; i += 2) {             if (n % i == 0) {                 // Update the prime factor                primeFactors[j++] = i;                while (n % i == 0)                    n /= i;            }        }         // If n is a prime number        // greater than 2        if (n > 2)            primeFactors[j++] = n;         return Arrays.copyOfRange(primeFactors, 0, j);    }     // Function that finds the shortest    // subsequence    private static void    findShortestSubsequence(int[] dp, int[] a,                            int index,                            int[] primeFactors)    {        int n = a.length;         for (int j = index; j < n; j++) {            int bitmask = 0;             for (int p = 0;                 p < primeFactors.length; p++) {                 // Check if the prime factor                // of first number, is also                // the prime factor of the                // rest numbers in array                if (a[j] % primeFactors[p] == 0) {                     // Set corresponding bit                    // of prime factor to 1,                    // it means both these                    // numbers have the                    // same prime factor                    bitmask ^= (1 << p);                }            }             for (int i = 0;                 i < dp.length; i++) {                 // If no states encountered                // so far continue for this                // combination of bits                if (dp[i] == n + 1)                    continue;                 // Update this state with                // minimum ways to reach                // this state                dp[bitmask & i]                    = Math.min(dp[bitmask & i],                               dp[i] + 1);            }        }    }     // Function that print the minimum    // length of subsequence    private static void    printMinimumLength(int[] a)    {        int min = a.length + 1;         for (int i = 0;             i < a.length - 1; i++) {             // Find the prime factors of            // the first number            int[] primeFactors                = findPrimeFactors(a[i]);             int n = primeFactors.length;             int[] dp = new int[1 << n];             // Initialize the array with            // maximum steps, size of the            // array + 1 for instance            Arrays.fill(dp, a.length + 1);             // Express the prime factors            // in bit representation             // Total number of set bits is            // equal to the total number            // of prime factors            int setBits = (1 << n) - 1;             // Indicates there is one            // way to reach the number            // under consideration            dp[setBits] = 1;            findShortestSubsequence(dp, a, i + 1,                                    primeFactors);             // State 0 corresponds            // to gcd of 1            min = Math.min(dp[0], min);        }         // If not found such subsequence        // then print "-1"        if (min == a.length + 1)            System.out.println(-1);         // Else print the length        else            System.out.println(min);    }     // Driver Code    public static void main(String[] args)    {        // Given array arr[]        int[] arr = { 2, 6, 12, 3 };         // Function Call        printMinimumLength(arr);    }}

## Python3

 # Python3 program for the above approach # Function that finds the prime# factors of a numberdef findPrimeFactors(n):         # To store the prime factor    primeFactors = [0 for i in range(9)]     j = 0     # 2s that divide n    if (n % 2 == 0):        primeFactors[j] = 2        j += 1                 while (n % 2 == 0):            n >>= 1     # N must be odd at this point    # Skip one element    i = 3    while (i * i <= n):        if (n % i == 0):                         # Update the prime factor            primeFactors[j] = i            j += 1                         while(n % i == 0):                n //= i                         i += 2     # If n is a prime number    # greater than 2    if (n > 2):        primeFactors[j] = n        j += 1     for i in range(0, j + 1):        primeFactors[i] = 0             return primeFactors # Function that finds the shortest# subsequencedef findShortestSubsequence(dp, a, index,                            primeFactors):    n = len(a)     for j in range(index, n):        bitmask = 0         for p in range(len(primeFactors)):                         # Check if the prime factor            # of first number, is also            # the prime factor of the            # rest numbers in array            if (primeFactors[p] != 0 and                a[j] % primeFactors[p] == 0):                                     # Set corresponding bit                # of prime factor to 1,                # it means both these                # numbers have the                # same prime factor                bitmask ^= (1 << p)         for i in range(len(dp)):                         # If no states encountered            # so far continue for this            # combination of bits            if (dp[i] == n + 1):                continue             # Update this state with            # minimum ways to reach            # this state            dp[bitmask & i] = min(dp[bitmask & i],                                  dp[i] + 1) # Function that print the minimum# length of subsequencedef printMinimumLength(a):         mn = len(a) + 1     for i in range(len(a) - 1):         # Find the prime factors of        # the first number        primeFactors = findPrimeFactors(a[i])         n = len(primeFactors)         dp = [0 for i in range(1 << n)]         # Initialize the array with        # maximum steps, size of the        # array + 1 for instance        dp = [len(a) + 1 for i in range(len(dp))]         # Express the prime factors        # in bit representation         # Total number of set bits is        # equal to the total number        # of prime factors        setBits = (1 << n) - 1         # Indicates there is one        # way to reach the number        # under consideration        dp[setBits] = 1                 findShortestSubsequence(dp, a, i + 1,                                primeFactors)         # State 0 corresponds        # to gcd of 1        mn = min(dp[0], mn)     # If not found such subsequence    # then print "-1"    if (mn == len(a) + 1):        print(-1)     # Else print the length    else:        print(mn) # Driver Codeif __name__ == '__main__':         # Given array arr[]    arr = [ 2, 6, 12, 3 ]     # Function Call    printMinimumLength(arr) # This code is contributed by bgangwar59



## C#

 // C# program for// the above approachusing System;class GFG{ // Function that finds the prime// factors of a numberprivate static int[] findPrimeFactors(int n){  // To store the prime factor  int[] primeFactors = new int[9];   int j = 0;   // 2s that divide n  if (n % 2 == 0)  {    primeFactors[j++] = 2;    while (n % 2 == 0)      n >>= 1;  }   // N must be odd at this point  // Skip one element  for (int i = 3;           i * i <= n; i += 2)  {    if (n % i == 0)    {      // Update the prime factor      primeFactors[j++] = i;      while (n % i == 0)        n /= i;    }  }   // If n is a prime number  // greater than 2  if (n > 2)    primeFactors[j++] = n;     int []temp = new int[j];  Array.Copy(primeFactors, temp, j);  return temp;} // Function that finds the shortest// subsequenceprivate static void findShortestSubsequence(int[] dp, int[] a,                                            int index,                                            int[] primeFactors){  int n = a.Length;   for (int j = index; j < n; j++)  {    int bitmask = 0;     for (int p = 0;             p < primeFactors.Length; p++)    {      // Check if the prime factor      // of first number, is also      // the prime factor of the      // rest numbers in array      if (a[j] % primeFactors[p] == 0)      {        // Set corresponding bit        // of prime factor to 1,        // it means both these        // numbers have the        // same prime factor        bitmask ^= (1 << p);      }    }     for (int i = 0;             i < dp.Length; i++)    {      // If no states encountered      // so far continue for this      // combination of bits      if (dp[i] == n + 1)        continue;       // Update this state with      // minimum ways to reach      // this state      dp[bitmask & i] = Math.Min(dp[bitmask & i],                                 dp[i] + 1);    }  }} // Function that print the minimum// length of subsequenceprivate static void printMinimumLength(int[] a){  int min = a.Length + 1;   for (int i = 0;           i < a.Length - 1; i++)  {    // Find the prime factors of    // the first number    int[] primeFactors = findPrimeFactors(a[i]);     int n = primeFactors.Length;     int[] dp = new int[1 << n];     // Initialize the array with    // maximum steps, size of the    // array + 1 for instance    for(i = 0; i < dp.Length; i++)      dp[i] = a.Length + 1;     // Express the prime factors    // in bit representation     // Total number of set bits is    // equal to the total number    // of prime factors    int setBits = (1 << n) - 1;     // Indicates there is one    // way to reach the number    // under consideration    dp[setBits] = 1;    findShortestSubsequence(dp, a, i + 1,                            primeFactors);     // State 0 corresponds    // to gcd of 1    min = Math.Min(dp[0], min);  }   // If not found such subsequence  // then print "-1"  if (min == a.Length + 1)    Console.WriteLine(-1);   // Else print the length  else    Console.WriteLine(min);} // Driver Codepublic static void Main(String[] args){  // Given array []arr  int[] arr = {2, 6, 12, 3};   // Function Call  printMinimumLength(arr);}} // This code is contributed by Rajput-Ji

Output

2

Time Complexity: O(N2)
Auxiliary Space: O(2^k) where k is the number of factors of array elements.