RemoveKDigits402.kt

package medium

import java.lang.StringBuilder
import java.text.CharacterIterator
import java.util.*

/**
 * 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 <= 105
num consists of only digits.
num does not have any leading zeros except for the zero itself.
 */
fun removeKdigits(num: String, k: Int): String {
    val size = num.length
    // edge case
    if (size == k) return "0"

    val stack = Stack<Int>()

    var counter = 0
    var i = 0

    while (counter < size) {
        while (i < k && stack.isNotEmpty() && stack.peek() > Character.getNumericValue(num[counter])) {
            stack.pop()
            i++
        }

        stack.push(Character.getNumericValue(num[counter++]))
    }
    while (i++ < k) {
        stack.pop()
    }
    val result = StringBuilder()

    while (stack.isNotEmpty()) {
        result.append(stack.pop())
    }


    result.reverse()

    while (result.length > 1 && result[0] == '0')
    {
        result.deleteCharAt(0)
    }

    return result.toString()
}