Algocademy Logo

Binary Strings With K Ones in Java (Time Complexity: O(N * K))


Understanding the Problem

The core challenge of this problem is to generate all possible binary strings of length N that contain exactly K ones. This problem is significant in combinatorics and has applications in coding theory, data compression, and error detection.

Potential pitfalls include misunderstanding the constraints or generating strings that do not meet the exact criteria of having K ones.

Approach

To solve this problem, we can use combinatorial mathematics. Specifically, we need to calculate the number of ways to choose K positions out of N to place the ones. This is a classic "combinations" problem, often denoted as C(N, K) or "N choose K".

We can use dynamic programming to efficiently compute the number of valid binary strings. The naive approach would involve generating all possible binary strings of length N and counting those with exactly K ones, but this is not optimal due to the exponential number of possible strings (2^N).

Instead, we can use a dynamic programming table where dp[i][j] represents the number of binary strings of length i with exactly j ones.

Algorithm

1. Initialize a 2D array dp where dp[i][j] is the number of binary strings of length i with exactly j ones.

2. Set dp[0][0] = 1 because there is exactly one way to have a string of length 0 with 0 ones (the empty string).

3. Iterate over the lengths from 1 to N and the number of ones from 0 to K.

4. For each position, update the dp table by considering two cases: adding a '0' or adding a '1'.

5. The final answer will be dp[N][K].

Code Implementation

public class BinaryStringsWithKOnes {
    public static int countBinaryStrings(int N, int K) {
        // Initialize the dp array
        int[][] dp = new int[N + 1][K + 1];
        
        // Base case: one way to have a string of length 0 with 0 ones
        dp[0][0] = 1;
        
        // Fill the dp table
        for (int i = 1; i <= N; i++) {
            for (int j = 0; j <= K; j++) {
                // Case 1: Add a '0' to strings of length i-1 with j ones
                dp[i][j] = dp[i - 1][j];
                
                // Case 2: Add a '1' to strings of length i-1 with j-1 ones
                if (j > 0) {
                    dp[i][j] += dp[i - 1][j - 1];
                }
            }
        }
        
        // The result is the number of binary strings of length N with exactly K ones
        return dp[N][K];
    }

    public static void main(String[] args) {
        int N = 4;
        int K = 2;
        System.out.println("Number of binary strings of length " + N + " with " + K + " ones: " + countBinaryStrings(N, K));
    }
}

Complexity Analysis

The time complexity of this approach is O(N * K) because we fill a table of size (N+1) * (K+1). The space complexity is also O(N * K) due to the storage required for the dp table.

Edge Cases

Consider the following edge cases:

  • N = 0, K = 0: The output should be 1 (the empty string).
  • K > N: The output should be 0 because it's impossible to have more ones than the length of the string.
  • K = 0: The output should be 1 (a string of all zeros).

Testing

To test the solution comprehensively, consider a variety of test cases:

  • Simple cases like N = 1, K = 0 and N = 1, K = 1.
  • Edge cases like N = 0, K = 0 and K > N.
  • More complex cases like N = 5, K = 3.

JUnit or any other testing framework can be used to automate these tests.

Thinking and Problem-Solving Tips

When approaching such problems, it's crucial to:

  • Understand the problem constraints and requirements thoroughly.
  • Break down the problem into smaller subproblems.
  • Consider both naive and optimized solutions.
  • Practice similar problems to improve problem-solving skills.

Conclusion

In this blog post, we discussed how to solve the problem of counting binary strings of length N with exactly K ones using dynamic programming. We covered the problem definition, approach, algorithm, code implementation, complexity analysis, edge cases, and testing. Understanding and solving such problems is crucial for developing strong problem-solving skills in computer science.

Additional Resources

For further reading and practice, consider the following resources: