Sometimes it is OK to try all permutations II

This one in particular only requires 3! permutations (also known as 6), the solution however is more generic for any number, although still an N! solution. Used Claude to help me refresh how to go from BigInteger to binary and vice-versa. Cheers, ACC

Maximum Possible Number by Binary Concatenation - LeetCode

3309. Maximum Possible Number by Binary Concatenation

Medium

You are given an array of integers nums of size 3.

Return the maximum possible number whose binary representation can be formed by concatenating the binary representation of all elements in nums in some order.

Note that the binary representation of any number does not contain leading zeros.

 

Example 1:

Input: nums = [1,2,3]

Output: 30

Explanation:

Concatenate the numbers in the order [3, 1, 2] to get the result "11110", which is the binary representation of 30.

Example 2:

Input: nums = [2,8,16]

Output: 1296

Explanation:

Concatenate the numbers in the order [2, 8, 16] to get the result "10100010000", which is the binary representation of 1296.

 

Constraints:

• nums.length == 3
• 1 <= nums[i] <= 127

public class Solution
{
    public int MaxGoodNumber(int[] nums)
    {
        BigInteger retVal = 0;

        MaxGoodNumber(nums, new HashSet(), "", ref retVal);
        return (int)retVal;
    }

    private void MaxGoodNumber(int[] nums,
                               HashSet usedIndex,
                               string currentNumberBinary,
                               ref BigInteger maxNumberBinary)
    {
        if (usedIndex.Count == nums.Length)
        {
            BigInteger num = BinaryStringToBigInteger(currentNumberBinary);
            if (num > maxNumberBinary) maxNumberBinary = num;
            return;
        }

        for (int i = 0; i < nums.Length; i++)
        {
            if (!usedIndex.Contains(i))
            {
                usedIndex.Add(i);
                MaxGoodNumber(nums, usedIndex, currentNumberBinary + Convert.ToString(nums[i], 2), ref maxNumberBinary);
                usedIndex.Remove(i);
            }
        }
    }

    public static BigInteger BinaryStringToBigInteger(string binary)
    {
        BigInteger result = 0;
        foreach (char c in binary)
        {
            result <<= 1;  // Shift left by 1
            if (c == '1')
                result += 1;
        }
        return result;
    }
}