#360 · #402 · Remove K Digits(移掉 K 位数字)

Given a non-negative integer as a string num and an integer k, remove k digits so that the remaining number is the smallest possible, preserving the relative order of the remaining digits. Return the result as a string with no leading zeros (use "0" if the result is empty). Constraints: 1 <= k <= num.length <= 10^5.