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