Description

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.

Hint

Train of Thought

stack,

    // k keeps track of how many characters we can remove
    // if the previous character in stk is larger than the current one
    // then removing it will get a smaller number
    // but we can only do so when k is larger than 0

Code

public class Solution {
    public String removeKdigits(String num, int k) {
        int len = num.length();
        //corner case
        if(k==len)        
            return "0";

        Stack<Character> stack = new Stack<>();
        int i =0;
        while(i<num.length()){
            //whenever meet a digit which is less than the previous digit, discard the previous one
            while(k>0 && !stack.isEmpty() && stack.peek()>num.charAt(i)){
                stack.pop();
                k--;
            }
            stack.push(num.charAt(i));
            i++;
        }

        // corner case like "1111"
        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();
    }
}

Complexity