Remove K Digits


#1

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.

#2

The below code uses a stack to remove k elements from the given string.

class Solution {
    public String removeKdigits(String num, int k) {
        if(k==num.length()) return "0";
        Stack<Character> stack = new Stack<>();
        int i=0;
        while(i<num.length()){
            while(k>0 && !stack.isEmpty() && stack.peek() > num.charAt(i)){
                stack.pop();
                k--;
            }
            stack.push(num.charAt(i));
            i++;
        }
        
        while(k>0){
            stack.pop();
            k--;
        }
        
        StringBuilder result = new StringBuilder();
        while(!stack.isEmpty()){
            result.append(stack.pop());
        }
        result.reverse();
        
        //remove all the 0 at the head
        while(result.length()>1 && result.charAt(0)=='0')
            result.deleteCharAt(0);
        
        return result.toString();
        
        
    }
}