Data Modelling & AI Data Structure & Algorithm

K-th Lexicographically smallest binary string with A 0s and B 1s

23 June 2025

0

Given three positive integers A, B, and K, the task is to find the K^th lexicographically smallest binary string that contains exactly A number of 0s and B number of 1s.

Example:

Input: A = 2, B = 2, K = 4
Output: 1001
Explanation: The lexicographic order of the strings is 0011, 0101, 0110, 1001.

Input: A = 3, B = 3, K = 7
Output: 010110

Approach: The above problem can be solved by using Dynamic Programming. Follow the below steps to solve this problem:

Initialize a 2D array dp[][] such that dp[i][j] will denote the total number of binary strings with i number of 0s and j number of 1s.
All the dp table values are initially filled with zeroes except dp[0][0] = 1 which denotes an empty string.
Now, dp[i][j] can be calculated as the sum of the total number of strings ending with 0(using the dp state as dp[i – 1][j]) and the string ending with 1(using the dp state as dp[i][j – 1]). So, the current dp state is calculated as dp[i][j] = dp[i – 1][j] + dp[i][j – 1].
After filling this dp table, a recursive function can be used to calculate the K^th lexicographically smallest binary string.
So, define a function KthString having parameters A, B, K, and dp.
Now, in each call of this recursive function:
- If the value of dp[A][B] is at least K then only ‘0’ can be present at this position in the Kth lexicographically smallest binary string and then recursively call function for the state (A – 1, B).
- Else ‘1’ is present here and recursively call function for the state (A, B – 1).
Print the answer according to the above observation.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Recursive function to find the Kth
// smallest binary string
string KthString(int A, int B, long long K,
                 vector<vector<int> >& dp)
{
    // Base Case
    if (A == 0) {
 
        // Return string of all 1's
        // of length B
        return string(B, '1');
    }
    if (B == 0) {
 
        // Return string of all 0's
        // of length A
        return string(A, '0');
    }
 
    if (K <= dp[A - 1][B]) {
        return "0" + KthString(
                         A - 1, B, K, dp);
    }
 
    else {
        return "1"
               + KthString(
                     A, B - 1,
                     K - dp[A - 1][B], dp);
    }
}
 
// Function to find the Kth lexicographically
// smallest binary string with exactly
// A zeroes and B ones
int KthStringUtil(int A, int B, int K)
{
    // Stores the recurring states
    vector<vector<int> > dp(
        A + 1, vector<int>(B + 1));
 
    // Calculate the dp values iteratively
    dp[0][0] = 1;
    for (int i = 0; i <= A; ++i) {
        for (int j = 0; j <= B; ++j) {
 
            if (i > 0) {
 
                // The last character was '0'
                dp[i][j] += dp[i - 1][j];
            }
            if (j > 0) {
 
                // The last character was '1'
                dp[i][j] += dp[i][j - 1];
            }
        }
    }
 
    // Print the binary string obtained
    cout << KthString(A, B, K, dp);
 
    return 0;
}
 
// Driver Code
int main()
{
    int A = 3, B = 3, K = 7;
    KthStringUtil(A, B, K);
 
    return 0;
}

Java

// Java program for the above approach
import java.io.*;
 
class GFG {
 
    // Recursive function to find the Kth
    // smallest binary string
    static String KthString(int A, int B, long K, int[][] dp)
    {
       
        // Base Case
        if (A == 0) {
 
            // Return string of all 1's
            // of length B
            String ans = "";
            for (int i = 0; i < B; i++) {
                ans += '1';
            }
            return ans;
        }
        if (B == 0) {
 
            // Return string of all 0's
            // of length A
            String ans = "";
            for (int i = 0; i < A; i++) {
                ans += '0';
            }
            return ans;
        }
 
        if (K <= dp[A - 1][B]) {
            return "0" + KthString(A - 1, B, K, dp);
        }
 
        else {
            return "1"
                + KthString(A, B - 1, K - dp[A - 1][B], dp);
        }
    }
 
    // Function to find the Kth lexicographically
    // smallest binary string with exactly
    // A zeroes and B ones
    static int KthStringUtil(int A, int B, int K)
    {
        // Stores the recurring states
        int[][] dp = new int[A + 1][B + 1];
 
        // Calculate the dp values iteratively
        dp[0][0] = 1;
        for (int i = 0; i <= A; ++i) {
            for (int j = 0; j <= B; ++j) {
 
                if (i > 0) {
 
                    // The last character was '0'
                    dp[i][j] += dp[i - 1][j];
                }
                if (j > 0) {
 
                    // The last character was '1'
                    dp[i][j] += dp[i][j - 1];
                }
            }
        }
 
        // Print the binary string obtained
        System.out.println(KthString(A, B, K, dp));
 
        return 0;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int A = 3, B = 3, K = 7;
        KthStringUtil(A, B, K);
    }
}
 
// This code is contributed by Dharanendra L V.

Python3

# Python Program to implement
# the above approach
 
# Recursive function to find the Kth
# smallest binary string
def KthString(A, B, K, dp):
   
    # Base Case
    if (A == 0):
 
        # Return string of all 1's
        # of length B
        str = ""
        for i in range(B):
            str += '1'
         
        return str
     
    if (B == 0):
 
        # Return string of all 0's
        # of length A
        str = ""
        for i in range(A):
            str += '0'
        return str
 
    if (K <= dp[A - 1][B]):
        return "0" + KthString( A - 1, B, K, dp)
 
    else:
        return "1" + KthString( A, B - 1, K - dp[A - 1][B], dp)
     
 
# Function to find the Kth lexicographically
# smallest binary string with exactly
# A zeroes and B ones
def KthStringUtil(A, B, K):
   
    # Stores the recurring states
    dp = [0] * (A + 1)
 
    for i in range(len(dp)):
        dp[i] = [0] * (B + 1)
     
 
    # Calculate the dp values iteratively
    dp[0][0] = 1
    for i in range(A + 1):
        for j in range(B + 1):
 
            if (i > 0):
 
                # The last character was '0'
                dp[i][j] += dp[i - 1][j]
             
            if (j > 0):
 
                # The last character was '1'
                dp[i][j] += dp[i][j - 1]
         
 
    # Print the binary string obtained
    print(KthString(A, B, K, dp))
 
# Driver Code
A = 3
B = 3
K = 7
KthStringUtil(A, B, K)
 
# This code is contributed by gfgking.

C#

// C# program for the above approach
using System;
class GFG {
 
    // Recursive function to find the Kth
    // smallest binary string
    static string KthString(int A, int B, long K,
                            int[, ] dp)
    {
 
        // Base Case
        if (A == 0) {
 
            // Return string of all 1's
            // of length B
            string ans = "";
            for (int i = 0; i < B; i++) {
                ans += '1';
            }
            return ans;
        }
        if (B == 0) {
 
            // Return string of all 0's
            // of length A
            string ans = "";
            for (int i = 0; i < A; i++) {
                ans += '0';
            }
            return ans;
        }
 
        if (K <= dp[A - 1, B]) {
            return "0" + KthString(A - 1, B, K, dp);
        }
 
        else {
            return "1"
                + KthString(A, B - 1, K - dp[A - 1, B], dp);
        }
    }
 
    // Function to find the Kth lexicographically
    // smallest binary string with exactly
    // A zeroes and B ones
    static int KthStringUtil(int A, int B, int K)
    {
        // Stores the recurring states
        int[, ] dp = new int[A + 1, B + 1];
 
        // Calculate the dp values iteratively
        dp[0, 0] = 1;
        for (int i = 0; i <= A; ++i) {
            for (int j = 0; j <= B; ++j) {
 
                if (i > 0) {
 
                    // The last character was '0'
                    dp[i, j] += dp[i - 1, j];
                }
                if (j > 0) {
 
                    // The last character was '1'
                    dp[i, j] += dp[i, j - 1];
                }
            }
        }
 
        // Print the binary string obtained
        Console.WriteLine(KthString(A, B, K, dp));
 
        return 0;
    }
 
    // Driver Code
    public static void Main(string[] args)
    {
        int A = 3, B = 3, K = 7;
        KthStringUtil(A, B, K);
    }
}
 
// This code is contributed by ukasp.

Javascript

<script>
        // JavaScript Program to implement
        // the above approach
 
        // Recursive function to find the Kth
        // smallest binary string
        function KthString(A, B, K,
            dp) {
            // Base Case
            if (A == 0) {
 
                // Return string of all 1's
                // of length B
                let str = "";
                for (let i = 0; i < B; i++) {
                    str += '1';
                }
                return str;
            }
            if (B == 0) {
 
                // Return string of all 0's
                // of length A
                let str = "";
                for (let i = 0; i < A; i++) {
                    str += '0';
                }
                return str;
 
            }
 
            if (K <= dp[A - 1][B]) {
                return "0" + KthString(
                    A - 1, B, K, dp);
            }
 
            else {
                return "1"
                    + KthString(
                        A, B - 1,
                        K - dp[A - 1][B], dp);
            }
        }
 
        // Function to find the Kth lexicographically
        // smallest binary string with exactly
        // A zeroes and B ones
        function KthStringUtil(A, B, K) {
            // Stores the recurring states
            let dp = new Array(A + 1);
 
            for (let i = 0; i < dp.length; i++) {
                dp[i] = new Array(B + 1).fill(0);
            }
 
            // Calculate the dp values iteratively
            dp[0][0] = 1;
            for (let i = 0; i <= A; ++i) {
                for (let j = 0; j <= B; ++j) {
 
                    if (i > 0) {
 
                        // The last character was '0'
                        dp[i][j] += dp[i - 1][j];
                    }
                    if (j > 0) {
 
                        // The last character was '1'
                        dp[i][j] += dp[i][j - 1];
                    }
                }
            }
 
            // Print the binary string obtained
            document.write(KthString(A, B, K, dp));
 
            return 0;
        }
 
        // Driver Code
        let A = 3, B = 3, K = 7;
        KthStringUtil(A, B, K);
 
     // This code is contributed by Potta Lokesh
 
    </script>

Output:

Time Complexity: O(A*B)
Auxiliary Space: O(A*B)

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K-th Lexicographically smallest binary string with A 0s and B 1s

C++

Java

Python3

C#

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