402. 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.

# @lc code=start
using LeetCode

function remove_kdigits(num::String, k::Int)::String
    num_code = codeunits(num)
    stk = UInt8[]
    for ch in num_code
        if k == 0 || isempty(stk)
            push!(stk, ch)
        else
            while !isempty(stk) && stk[end] > ch && k > 0
                pop!(stk)
                k -= 1
            end
            push!(stk, ch)
        end
    end
    idx = findfirst(x -> x != UInt8('0'), stk)
    if idx === nothing
        return "0"
    end
    return String(@view stk[idx:(end - k)])
end
# @lc code=end

remove_kdigits (generic function with 1 method)

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