Remove K Digits

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.

• The given num does not contain any leading zero.

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.

public class Solution {
    // MSP -> Most Significant Position
    // The theory is that Reducing digits at MSP is better than reducing digit at LSP
    public String removeKdigits(String num, int k) {
        // corner case
        if (k == num.length())
            return "0";

        Deque<Character> stack = new LinkedList<>();
        int i = 0;
        // Starting from MSP -> LSP
        for (char c : num.toCharArray()) {
            while (k > 0 && !stack.isEmpty() && stack.peek() > c) {
                stack.pop();
                k--;
            }
            stack.push(c);
        }
        // corner case like "11234"
        // Remove digits from the end(bcoz they are in inc order)
        while (k > 0) {
            stack.pop();
            k--;
        }
        // construct the number from the stack
        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty())
            sb.append(stack.pop());
        sb.reverse();
        // remove all the 0 at the head
        while (sb.length() > 1 && sb.charAt(0) == '0')
            sb.deleteCharAt(0);
        return sb.toString();
    }
}