402. Remove K Digits

Problem

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

  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.

  Constraints:

• 1 <= k <= num.length <= 10^5

• num consists of only digits.

• num does not have any leading zeros except for the zero itself.

Solution

class Solution {
    public String removeKdigits(String num, int k) {
        char[] list = new char[num.length()];
        int len = num.length() - k;
        int top = 0;
        for (int i = 0; i < num.length(); i++) {
            while (top > 0 && k > 0 && num.charAt(i) < list[top - 1]) {
                top--;
                k--;
            }
            list[top++] = num.charAt(i);
        }
        int number = 0;
        while (number < len && list[number] == '0') {
            number++;
        }
        return number == len ? "0" : new String(list, number, len - number);
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).