Tags: "leetcode", "stack", "greedy", access_time 2-min read

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Remove K Digits

Created: May 14, 2020 by [lek-tin]

Last updated: May 14, 2020

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

Java

class Solution {
    public String removeKdigits(String num, int k) {
        LinkedList<Character> stack = new LinkedList<Character>();

        for(char digit : num.toCharArray()) {
            while(stack.size() > 0 && k > 0 && stack.peekLast() > digit) {
                stack.removeLast();
                k -= 1;
            }
            stack.addLast(digit);
        }

        /* remove the remaining digits from the tail. */
        for(int i=0; i<k; ++i) {
            stack.removeLast();
        }

        // build the final string, while removing the leading zeros.
        StringBuilder res = new StringBuilder();
        boolean leadingZero = true;
        for(char digit: stack) {
        if (leadingZero && digit == '0') continue;
            leadingZero = false;
            res.append(digit);
        }

        /* return the final string  */
        if (res.length() == 0) return "0";
        return res.toString();
    }
}