May Leetcode Challenge - Day 13

Remove K Digits

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

This problem is hard. Solution for each k character, remove the largest one. Reduce K for next step
Don't forget to remove leading zeroes

class Solution {
    func removeKdigits(_ num: String, _ k: Int) -> String {
        var k = k
        var stack = [Character]()
        for c in num {
            while k > 0, let top = stack.last, top > char {
                stack.removeLast()
                k -= 1
            }
            stack.append(char)
        }
        stack.removeLast(k)
        let result = stack.drop(while: { $0 == "0" })
        return result.isEmpty ? "0" : String(result)
    }
}