May-13 2020 Challenge

13. 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.

Solution in java :

class Solution {

    public String removeKdigits(String num, int k) {

        if (num.length() == k)

            return "0";

        StringBuilder sb = new StringBuilder(num);

        for (int j = 0; j < k; j++) {

            int i = 0;

            while (i < sb.length() - 1 && sb.charAt(i) <= sb.charAt(i + 1)) {

                i++;

            }

            sb.delete(i, i + 1);

        }

        while (sb.length() > 1 && sb.charAt(0) == '0')

            sb.delete(0, 1);

        if (sb.length() == 0) {

            return "0";

        }

        return sb.toString();

    }

}

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