LeetCode - Remove K Digits

Question Definition

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.
  

  Java Solution

  public String removeKdigits(String num, int k) {
    int remain = num.length() - k;
    StringBuilder sb = new StringBuilder();
    for(char c : num.toCharArray()){
        while(sb.length() > 0 && k > 0 && sb.charAt(sb.length() - 1) > c){
            sb.deleteCharAt(sb.length() - 1);
            k--;
        }
        sb.append(c);
    }
  
    sb.delete(remain, sb.length());
    while (sb.length() > 0 && sb.charAt(0) == '0') sb.deleteCharAt(0);
    return sb.length() == 0 ? "0" : sb.toString();
  }