RemoveKDigits.java

#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 {
    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);
        }
        int i = 0;
        while (i < sb.length() - 1 && sb.charAt(i) == '0') {
            i ++;
        }
        return sb.toString().substring(i);
    }
}