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Count ways to divide circle using N non-intersecting chord | Set-2
  • Last Updated : 26 Aug, 2019

Given a number N. The task is to find the number of ways you can draw N chords in a circle with 2*N points such that no two chords intersect. Two ways are different if there exists a chord which is present in one way and not in other. As the answer could be large print it modulo 10^9+7.

Examples:

Input : N = 2
Output : 2
If points are numbered 1 to 4 in clockwise direction,
then different ways to draw chords are:
{(1-2), (3-4)} and {(1-4), (2-3)}

Input :N = 1
Output : 1

Approach:
If we draw a chord between any two points, the current set of points getting broken into two smaller sets S_1 and S_2. If we draw a chord from a point in S_1 to a point in S_2, it will surely intersect the chord we’ve just drawn. So, we can arrive at a recurrence that:



Ways(n) = sum[i = 0 to n-1] { Ways(i)*Ways(n-i-1) }.

The above recurrance relation is similiar to the recurrance relation for nth Catalan number which is equal to 2nCn / (n+1) . Instead of dividing the numeration with the denomination, multiply the numberator with the modulo inverse of the denominator as division is not allowed in modulo domain.

Below is the implementation of the above approach:

C++




// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to calculate x^y %mod efficiently
int power(long long x, int y, int mod)
{
  
    // Initialize the answer
    long long res = 1;
    while (y) {
  
        // If power is odd
        if (y & 1)
  
            // Update the answer
            res = (res * x) % mod;
  
        // Square the base and half the exponent
        x = (x * x) % mod;
        y = (y >> 1);
    }
  
    // Return the value
    return (int)(res % mod);
}
  
  
  
// Function to calculate ncr%mod efficiently
int ncr(int n, int r, int mod)
{
  
    // Initialize the answer
    long long res = 1;
  
    // Calculate ncr in O(r)
    for (int i = 1; i <= r; i += 1) {
  
        // Multiply with the numerator factor
        res = (res * (n - i + 1)) % mod;
  
        // Calculate the inverse of factor of denominator
        int inv = power(i, mod - 2, mod);
  
        // Multiply with inverse value
        res = (res * inv) % mod;
    }
  
    // Return answer value
    return (int)(res%mod);
}
  
// Function to return the number
// of non intersecting chords
int NoOfChords(int A)
{
  
    // define mod value
    int mod = 1e9 + 7;
  
    // Value of C(2n, n)
    long long ans = ncr(2 * A, A, mod);
  
    // Modulo inverse of (n+1)
    int inv = power(A + 1, mod - 2, mod);
  
    // Multiply with modulo inverse
    ans = (ans * inv) % mod;
  
    // Return the answer
    return (int)(ans%mod);
}
  
// Driver code
int main()
{
  
    int N = 2;
      
    // Function call
    cout << NoOfChords(N);
  
    return 0;
}

Java




// Java implementation of the approach
class GFG
{
  
    // Function to calculate x^y %mod efficiently 
    static int power(long x, int y, int mod) 
    
      
        // Initialize the answer 
        long res = 1
        while (y != 0)
        
      
            // If power is odd 
            if ((y & 1) == 1
      
                // Update the answer 
                res = (res * x) % mod; 
      
            // Square the base and half the exponent 
            x = (x * x) % mod; 
            y = (y >> 1); 
        
      
        // Return the value 
        return (int)(res % mod); 
    
      
    // Function to calculate ncr%mod efficiently 
    static int ncr(int n, int r, int mod) 
    
      
        // Initialize the answer 
        long res = 1
      
        // Calculate ncr in O(r) 
        for (int i = 1; i <= r; i += 1
        
      
            // Multiply with the numerator factor 
            res = (res * (n - i + 1)) % mod; 
      
            // Calculate the inverse of 
            // factor of denominator 
            int inv = power(i, mod - 2, mod); 
      
            // Multiply with inverse value 
            res = (res * inv) % mod; 
        
      
        // Return answer value 
        return (int)(res % mod); 
    
      
    // Function to return the number 
    // of non intersecting chords 
    static int NoOfChords(int A) 
    
      
        // define mod value 
        int mod = (int)(1e9 + 7); 
      
        // Value of C(2n, n) 
        long ans = ncr(2 * A, A, mod); 
      
        // Modulo inverse of (n+1) 
        int inv = power(A + 1, mod - 2, mod); 
      
        // Multiply with modulo inverse 
        ans = (ans * inv) % mod; 
      
        // Return the answer 
        return (int)(ans % mod); 
    
      
    // Driver code 
    public static void main(String[] args) 
    {
        int N = 2
      
        // Function call 
        System.out.println(NoOfChords(N));
    }
}
  
// This code is contributed by 29AjayKumar

Python3




# Python3 implementation of the above approach
  
# Function to calculate x^y %mod efficiently
def power(x, y, mod):
  
    # Initialize the answer
    res = 1
    while (y):
  
        # If power is odd
        if (y & 1):
  
            # Update the answer
            res = (res * x) % mod
  
        # Square the base and half the exponent
        x = (x * x) % mod
        y = (y >> 1)
  
  
    # Return the value
    return (res % mod)
  
# Function to calculate ncr%mod efficiently
def ncr(n, r, mod):
  
  
    # Initialize the answer
    res = 1
  
    # Calculate ncr in O(r)
    for i in range(1,r+1):
  
        # Multiply with the numerator factor
        res = (res * (n - i + 1)) % mod
  
        # Calculate the inverse of factor of denominator
        inv = power(i, mod - 2, mod)
  
        # Multiply with inverse value
        res = (res * inv) % mod
  
  
    # Return answer value
    return (res%mod)
  
# Function to return the number
# of non intersecting chords
def NoOfChords(A):
  
  
    # define mod value
    mod = 10**9 + 7
  
    # Value of C(2n, n)
    ans = ncr(2 * A, A, mod)
  
    # Modulo inverse of (n+1)
    inv = power(A + 1, mod - 2, mod)
  
    # Multiply with modulo inverse
    ans = (ans * inv) % mod
  
    # Return the answer
    return (ans%mod)
  
  
# Driver code
  
N = 2
  
# Function call
print(NoOfChords(N))
  
# This code is contributed by mohit kumar 29

C#




// Java implementation of the above approach 
using System;
  
class GFG 
{
  
    // Function to calculate x^y %mod efficiently 
    static int power(long x, int y, int mod) 
    
      
        // Initialize the answer 
        long res = 1; 
        while (y != 0)
        
      
            // If power is odd 
            if ((y & 1) == 1) 
      
                // Update the answer 
                res = (res * x) % mod; 
      
            // Square the base and half the exponent 
            x = (x * x) % mod; 
            y = (y >> 1); 
        
      
        // Return the value 
        return (int)(res % mod); 
    
      
    // Function to calculate ncr%mod efficiently 
    static int ncr(int n, int r, int mod) 
    
      
        // Initialize the answer 
        long res = 1; 
      
        // Calculate ncr in O(r) 
        for (int i = 1; i <= r; i += 1) 
        
      
            // Multiply with the numerator factor 
            res = (res * (n - i + 1)) % mod; 
      
            // Calculate the inverse of factor of denominator 
            int inv = power(i, mod - 2, mod); 
      
            // Multiply with inverse value 
            res = (res * inv) % mod; 
        
      
        // Return answer value 
        return (int)(res % mod); 
    
      
    // Function to return the number 
    // of non intersecting chords 
    static int NoOfChords(int A) 
    
      
        // define mod value 
        int mod = (int)(1e9 + 7); 
      
        // Value of C(2n, n) 
        long ans = ncr(2 * A, A, mod); 
      
        // Modulo inverse of (n+1) 
        int inv = power(A + 1, mod - 2, mod); 
      
        // Multiply with modulo inverse 
        ans = (ans * inv) % mod; 
      
        // Return the answer 
        return (int)(ans % mod); 
    
      
    // Driver code 
    public static void Main () 
    {
        int N = 2; 
          
        // Function call 
        Console.WriteLine(NoOfChords(N));
    }
}
  
// This code is contributed by AnkitRai01
Output:
2

Time complexity : O(N*log(mod))

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