[LeetCode][Kotlin] 402. Remove K Digits

402. Remove K Digits

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

음이 아닌 정수 num을 나타내는 문자열 num과 정수 k가 주어지면, num에서 k자리를 제거한 후 가능한 가장 작은 정수를 반환합니다.

 

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.

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코드

class Solution {
    fun removeKdigits(num: String, k: Int): String {
        val stack = mutableListOf<Char>()
        var k = k

        num.forEach { c ->
            while (k > 0 && stack.isNotEmpty() && stack.last() > c) {
                k -= 1
                stack.removeLast()
            }
            stack.add(c)
        }

        val newStack = stack.slice(0..stack.lastIndex - k)
        val res = newStack.joinToString("").trimStart('0')
        return if (res != "") res else "0"
    }
}