Largest number in K swaps

Given a number K and string str of digits denoting a positive integer, build the largest number possible by performing swap operations on the digits of str at most K times.

• Time : O(n^2 * k!)
• Space : O(1)

class Solution {
public:
    //Function to find the largest number after k swaps.
    string findMaximumNum(string& str, int k, int pos = 0) {
        if(pos >= str.length() or k == 0) return str;
        string res = str;
        for(int i = pos + 1; i < str.length(); i++) {
            if(str[i] > str[pos]) {
                swap(str[i], str[pos]);
                res = max(res, findMaximumNum(str, k - 1, pos + 1));
                swap(str[i], str[pos]);
            }
        }
        
        return max(res, findMaximumNum(str, k, pos + 1));
    }
};