Given a number k and string num of digits (0 to 9) denoting a positive integer. Find a string denoting the lowest integer number possible by removing k digits from num, without changing their order.
Note: num will not contain any leading zero.

Example 1:
Input:
k = 2
num = "143729"
Output: "1329"
Explanation: 1329 is the minimum number
possible after removing '4' and '7'.

Example 2:
Input:
k = 3
num = "10056"
Output: "0"
Explanation: 0 is the minimum number
possible after removing '1' , '5' and '6'.

Your Task:
You dont need to read input or print anything. Complete the function buildLowestNumber() which accepts string num and integer k as input parameters and returns a string denoting the smallest integer possible after removing k digits from num without changing the order.
Expected Time Complexity: O(Length of num)
Expected Auxiliary Space: O(Length of num)
Constraints:
1 <= Length of num <= 10^{5}
0 <= k < Length of num

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