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Find all even length binary sequences with same sum of first and second half bits
  • Difficulty Level : Medium
  • Last Updated : 27 May, 2019

Given a number n, find all binary sequences of length 2n such that sum of first n bits is same as sum of last n bits.

Examples:

Input:  N = 2
Output: 
0101 1111 1001 0110 0000 1010 

Input:  N = 3
Output:  
011011 001001 011101 010001 101011 111111
110011 101101 100001 110101 001010 011110 
010010 001100 000000 010100 101110 100010 
110110 100100 

The idea is to fix first and last bits and then recur for remaining 2*(n-1) bits. There are four possibilities when we fix first and last bits –

  1. First and last bits are 1, remaining n – 1 bits on both sides should also have the same sum.
  2. First and last bits are 0, remaining n – 1 bits on both sides should also have the same sum.
  3. First bit is 1 and last bit is 0, sum of remaining n – 1 bits on left side should be 1 less than the sum n-1 bits on right side.
  4. First bit is 0 and last bit is 1, sum of remaining n – 1 bits on left side should be 1 more than the sum n-1 bits on right side.

Below is implementation of above idea –



C++




// C++ program to print even length binary sequences
// whose sum of first and second half bits is same
#include <bits/stdc++.h>
using namespace std;
  
// Function to print even length binary sequences
// whose sum of first and second half bits is same
  
// diff --> difference between sums of first n bits
// and last n bits
// out --> output array
// start --> starting index
// end --> ending index
void findAllSequences(int diff, char* out, int start, int end)
{
    // We can't cover difference of more than n with 2n bits
    if (abs(diff) > (end - start + 1) / 2)
        return;
  
    // if all bits are filled
    if (start > end)
    {
        // if sum of first n bits and last n bits are same
        if (diff == 0)
            cout << out << " ";
        return;
    }
  
    // fill first bit as 0 and last bit as 1
    out[start] = '0', out[end] = '1';
    findAllSequences(diff + 1, out, start + 1, end - 1);
  
    // fill first and last bits as 1
    out[start] = out[end] = '1';
    findAllSequences(diff, out, start + 1, end - 1);
  
    // fill first and last bits as 0
    out[start] = out[end] = '0';
    findAllSequences(diff, out, start + 1, end - 1);
  
    // fill first bit as 1 and last bit as 0
    out[start] = '1', out[end] = '0';
    findAllSequences(diff - 1, out, start + 1, end - 1);
}
  
// Driver program
int main()
{
    // input number
    int n = 2;
  
    // allocate string contaning 2*n characters
    char out[2 * n + 1];
  
    // null terminate output array
    out[2 * n] = '\0';
  
    findAllSequences(0, out, 0, 2*n - 1);
  
    return 0;
}

Java




// Java program to print even length binary 
// sequences whose sum of first and second
// half bits is same
import java.io.*;
import java.util.*;
  
class GFG 
{
    // Function to print even length binary sequences
    // whose sum of first and second half bits is same
   
    // diff --> difference between sums of first n bits
    // and last n bits
    // out --> output array
    // start --> starting index
    // end --> ending index
    static void findAllSequences(int diff, char out[], 
                                     int start, int end)
    {
        // We can't cover difference of more 
        // than n with 2n bits
        if (Math.abs(diff) > (end - start + 1) / 2)
            return;
   
        // if all bits are filled
        if (start > end)
        {
            // if sum of first n bits and
            // last n bits are same
            if (diff == 0)
            {
                System.out.print(out);
                System.out.print(" ");
            }    
            return;
        }
   
        // fill first bit as 0 and last bit as 1
        out[start] = '0';
        out[end] = '1';
        findAllSequences(diff + 1, out, start + 1, end - 1);
   
        // fill first and last bits as 1
        out[start] = out[end] = '1';
        findAllSequences(diff, out, start + 1, end - 1);
   
        // fill first and last bits as 0
        out[start] = out[end] = '0';
        findAllSequences(diff, out, start + 1, end - 1);
   
        // fill first bit as 1 and last bit as 0
        out[start] = '1';
        out[end] = '0';
        findAllSequences(diff - 1, out, start + 1, end - 1);
    }
      
    // Driver program
    public static void main (String[] args) 
    {
        // input number
        int n = 2;
   
        // allocate string contaning 2*n characters
        char[] out = new char[2 * n + 1];
   
        // null terminate output array
        out[2 * n] = '\0';
   
        findAllSequences(0, out, 0, 2*n - 1);
    }
}
  
// This code is contributed by Pramod Kumar

Python3




# Python3 program to print even length binary sequences
# whose sum of first and second half bits is same
  
# Function to print even length binary sequences
# whose sum of first and second half bits is same
  
# diff --> difference between sums of first n bits
# and last n bits
# out --> output array
# start --> starting index
# end --> ending index
def findAllSequences(diff, out, start, end):
  
    # We can't cover difference of more than n with 2n bits
    if (abs(diff) > (end - start + 1) // 2):
        return;
  
    # if all bits are filled
    if (start > end):
        # if sum of first n bits and last n bits are same
        if (diff == 0):
            print(''.join(list(out)),end=" ");
        return;
  
    # fill first bit as 0 and last bit as 1
    out[start] = '0';
    out[end] = '1';
    findAllSequences(diff + 1, out, start + 1, end - 1);
  
    # fill first and last bits as 1
    out[start] = out[end] = '1';
    findAllSequences(diff, out, start + 1, end - 1);
  
    # fill first and last bits as 0
    out[start] = out[end] = '0';
    findAllSequences(diff, out, start + 1, end - 1);
  
    # fill first bit as 1 and last bit as 0
    out[start] = '1';
    out[end] = '0';
    findAllSequences(diff - 1, out, start + 1, end - 1);
  
# Driver program
  
# input number
n = 2;
  
# allocate string contaning 2*n characters
out=[""]*(2*n);
  
findAllSequences(0, out, 0, 2*n - 1);
  
# This code is contributed by mits

C#




// C# program to print even length binary 
// sequences whose sum of first and second
// half bits is same
using System;
  
class GFG {
      
    // Function to print even length binary
    // sequences whose sum of first and 
    // second half bits is same
  
    // diff --> difference between sums of
    // first n bits
    // and last n bits
    // out --> output array
    // start --> starting index
    // end --> ending index
    static void findAllSequences(int diff,
            char []outt, int start, int end)
    {
          
        // We can't cover difference of 
        // more than n with 2n bits
        if (Math.Abs(diff) > (end - start
                                   + 1) / 2)
            return;
  
        // if all bits are filled
        if (start > end)
        {
              
            // if sum of first n bits and
            // last n bits are same
            if (diff == 0)
            {
                Console.Write(outt);
                Console.Write(" ");
            
            return;
        }
  
        // fill first bit as 0 and last bit
        // as 1
        outt[start] = '0';
        outt[end] = '1';
        findAllSequences(diff + 1, outt, 
                        start + 1, end - 1);
  
        // fill first and last bits as 1
        outt[start] = outt[end] = '1';
        findAllSequences(diff, outt, 
                        start + 1, end - 1);
  
        // fill first and last bits as 0
        outt[start] = outt[end] = '0';
        findAllSequences(diff, outt, 
                         start + 1, end - 1);
  
        // fill first bit as 1 and last 
        // bit as 0
        outt[start] = '1';
        outt[end] = '0';
        findAllSequences(diff - 1, outt,
                         start + 1, end - 1);
    }
      
    // Driver program
    public static void Main () 
    {
          
        // input number
        int n = 2;
  
        // allocate string contaning 2*n 
        // characters
        char []outt = new char[2 * n + 1];
  
        // null terminate output array
        outt[2 * n] = '\0';
  
        findAllSequences(0, outt, 0, 2*n - 1);
    }
}
  
// This code is contributed by nitin mittal.

PHP




<?php
// PHP program to print even length binary sequences
// whose sum of first and second half bits is same
  
// Function to print even length binary sequences
// whose sum of first and second half bits is same
  
// diff --> difference between sums of first n bits
// and last n bits
// out --> output array
// start --> starting index
// end --> ending index
function findAllSequences($diff, $out, $start, $end)
{
    // We can't cover difference of more than n with 2n bits
    if (abs($diff) > (int)(($end - $start + 1) / 2))
        return;
  
    // if all bits are filled
    if ($start > $end)
    {
        // if sum of first n bits and last n bits are same
        if ($diff == 0)
            print(implode("",$out)." ");
        return;
    }
  
    // fill first bit as 0 and last bit as 1
    $out[$start] = '0';
    $out[$end] = '1';
    findAllSequences($diff + 1, $out, $start + 1, $end - 1);
  
    // fill first and last bits as 1
    $out[$start] = $out[$end] = '1';
    findAllSequences($diff, $out, $start + 1, $end - 1);
  
    // fill first and last bits as 0
    $out[$start] = $out[$end] = '0';
    findAllSequences($diff, $out, $start + 1, $end - 1);
  
    // fill first bit as 1 and last bit as 0
    $out[$start] = '1';
    $out[$end] = '0';
    findAllSequences($diff - 1, $out, $start + 1, $end - 1);
}
  
// Driver program
  
    // input number
    $n = 2;
  
    // allocate string contaning 2*n characters
    $out=array_fill(0,2*$n,"");
  
    findAllSequences(0, $out, 0, 2*$n - 1);
  
// This code is contributed by chandan_jnu
?>


Output:
0101 1111 1001 0110 0000 1010 

This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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