Largest number in K swaps.cpp

/*
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.

Example 1:
Input:
K = 4
str = "1234567"
Output:
7654321
Explanation:
Three swaps can make the
input 1234567 to 7654321, swapping 1
with 7, 2 with 6 and finally 3 with 5

Example 2:
Input:
K = 3
str = "3435335"
Output:
5543333
Explanation:
Three swaps can make the input
3435335 to 5543333, swapping 3 
with 5, 4 with 5 and finally 3 with 4 

Your task:
You don't have to read input or print anything. Your task is to complete the function findMaximumNum() which takes the string and an integer as input and returns a string containing the largest number formed by perfoming the swap operation at most k times.

Expected Time Complexity: O(n!/(n-k)!) , where n = length of input string
Expected Auxiliary Space: O(n)

Constraints:
1 ≤ |str| ≤ 30
1 ≤ K ≤ 10
*/

void findmax(string str, string &ans, int k, int n, int i)
{
  if (k == 0)
    return;

  char maxs = str[i];

  for (int j = i + 1; j < n; j++)
  {
    maxs = max(maxs, str[j]);
  }

  if (maxs != str[i])
    k--;
  for (int j = i; j < n; j++)
  {
    if (str[j] == maxs)
    {
      swap(str[i], str[j]);
      ans = max(ans, str);
      findmax(str, ans, k, n, i + 1);
      swap(str[i], str[j]);
    }
  }
  return;
}

string findMaximumNum(string str, int k)
{
  if (k == 0)
    return str;
  string ans = "";
  findmax(str, ans, k, str.size(), 0);
  return ans;
}