Binary Strings With K Ones On Even Positions - Time Complexity: O(N choose K) - JavaScript

Given two non-negative integers N and K, return the number of binary strings of length N with exactly K ones placed only on even positions

Example:

Input: N = 6, K = 2
Output: 3
Explanation: [
    "101000",
    "100010",
    "001010"
]

Notes: 2 * K <= N <= 30

Understanding the Problem

The core challenge of this problem is to generate binary strings of a given length N such that exactly K ones are placed at even positions. This problem is significant in combinatorial generation and has applications in areas like coding theory and binary sequence analysis.

Potential pitfalls include misunderstanding the even positions (0-based indexing) and ensuring that the number of ones does not exceed the available even positions.

Approach

To solve this problem, we need to consider the following steps:

Identify the even positions in a binary string of length N.
Determine the number of ways to place exactly K ones in these even positions.

A naive solution would involve generating all possible binary strings of length N and counting those that meet the criteria. However, this is not optimal due to the exponential number of possible strings.

An optimized approach involves combinatorial mathematics. Specifically, we can use the binomial coefficient to count the number of ways to choose K positions out of the available even positions.

Algorithm

Here is a step-by-step breakdown of the algorithm:

Calculate the number of even positions in a binary string of length N. This is given by Math.floor(N / 2).
Use the binomial coefficient formula to compute the number of ways to choose K positions out of these even positions.

Code Implementation

// Function to calculate binomial coefficient
function binomialCoefficient(n, k) {
    if (k > n) return 0;
    if (k === 0 || k === n) return 1;
    let res = 1;
    for (let i = 0; i < k; i++) {
        res *= (n - i);
        res /= (i + 1);
    }
    return res;
}

// Main function to count binary strings with K ones on even positions
function countBinaryStrings(N, K) {
    // Calculate the number of even positions
    const evenPositions = Math.floor(N / 2);
    
    // Calculate the number of ways to place K ones in these even positions
    return binomialCoefficient(evenPositions, K);
}

// Example usage
const N = 6;
const K = 2;
console.log(countBinaryStrings(N, K)); // Output: 3

Complexity Analysis

The time complexity of the binomial coefficient calculation is O(K), as it involves a loop that runs K times. The space complexity is O(1) since we are using a constant amount of extra space.

Edge Cases

Potential edge cases include:

N is very small (e.g., 0 or 1).
K is 0, which should always return 1 (the empty set).
K is greater than the number of available even positions, which should return 0.

Examples:

N = 4, K = 0 -> Output: 1
N = 4, K = 3 -> Output: 0

Testing

To test the solution comprehensively, consider the following test cases:

Simple cases with small N and K.
Cases where K is 0 or greater than the number of even positions.
Edge cases with the maximum constraints.

Example test cases:

console.log(countBinaryStrings(6, 2)); // Output: 3
console.log(countBinaryStrings(4, 1)); // Output: 2
console.log(countBinaryStrings(4, 0)); // Output: 1
console.log(countBinaryStrings(4, 3)); // Output: 0

Thinking and Problem-Solving Tips

When approaching such problems, consider the following tips:

Break down the problem into smaller, manageable parts.
Think about combinatorial mathematics and how it can simplify counting problems.
Practice similar problems to improve your understanding and problem-solving skills.

Conclusion

In this blog post, we discussed how to count binary strings of length N with exactly K ones placed at even positions. We explored a combinatorial approach to solve the problem efficiently and provided a detailed explanation of the algorithm and its implementation in JavaScript.

Understanding and solving such problems is crucial for developing strong problem-solving skills and a deep understanding of combinatorial mathematics.

Additional Resources

For further reading and practice, consider the following resources: