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.
class Solution(object):
     def removeKdigits(self, num, k):
         """
         :type num: str
         :type k: int
         :rtype: str
         """
         if k == len(num):
             return "0"
         stack = []
         cnt = 0
         for i, ch in enumerate(num):
             if cnt == k:
                 for j in range(i, len(num)):
                     stack.append(num[j])
                 break
             if not stack or ch >= stack[-1]:
                 stack.append(ch)
             else:
                 while stack and ch < stack[-1] and cnt < k:
                     stack.pop()
                     cnt += 1
                 stack.append(ch)
         while cnt < k:
             stack.pop()
             cnt += 1
         i = 0
         while i < len(stack) and stack[i] == '0':
             i += 1
         if i == len(stack):
             return "0"
         return ''.join(stack[i:])
class Solution(object):
     def removeKdigits(self, num, k):
         """
         :type num: str
         :type k: int
         :rtype: str
         """
         if k == len(num):
             return "0"
         stack = []
         cnt = 0
         for i, ch in enumerate(num):
             if cnt == k:
                 for j in range(i, len(num)):
                     stack.append(num[j])
                 break
             if not stack or ch >= stack[-1]:
                 stack.append(ch)
             else:
                 while stack and ch < stack[-1] and cnt < k:
                     stack.pop()
                     cnt += 1
                 stack.append(ch)
         while cnt < k:
             stack.pop()
             cnt += 1
         i = 0
         while i < len(stack) and stack[i] == '0':
             i += 1
         if i == len(stack):
             return "0"
         return ''.join(stack[i:])