402. Remove K Digits

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

Constraints:

1 <= k <= num.length <= 105
num consists of only digits.
num does not have any leading zeros except for the zero itself.

Solution:

We need to make the smallest number by removing K digits. So we will remove the digits that are largest from the left.
Ex- if the num is "654" and k is 1 then we will remove 6

if the num is "456" and k is 1 then we will remove 6

We will keep a stack and whenever the current digit is smaller than the top of the stack we will pop the stack until k>0 and the stack is not empty and the current digit is smaller than the top.

Another condition is that the current digit is 0 and the size of the stack is 1 and only contains 0 so we need to pop it.

If we have iterated over num and still k>0 then we need to pop the stack until k>0.
Now we can get our answer easily.

See the code below for better understanding.