[Algorithm] The smallest possible integer after removing k

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.

public class Solution {
    public string RemoveKdigits(string num, int k) {
        char[] result = new char[num.Length];
        int top = 0;
        
        foreach (char c in num){
            // Remove digits from the stack as long as they are greater than current digit
            // and there are still digits to remove
            while(top > 0 && result[top - 1] > c && k > 0) {
                top--;
                k--;
            }
            
            // Add current digit to the stack
            result[top++] = c;
        }
        
        // If there are still digits to remove, remove them from the end
        while (k > 0 && top > 0) {
            top--;
            k--;
        }
        
        // Trim leading zeros
        int start = 0;
        while (start < top && result[start] == '0') {
            start++;
        }
        
        // Construct the final result string
        return start == top ? "0" : new string(result, start, top - start);
    }
}