题目
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.
解题思路
贪心，尽量使得这个数是不降的，使用一个栈来维护，如果已经是不降的了，那么就从最后开始删
代码
class Solution:
def removeKdigits(self, num: str, k: int) -> str:
if k == len(num):
return '0'
stack = []
for n in num:
while k and stack and n < stack[-1]:
stack.pop()
k -= 1
stack.append(n)
for i in range(k):
stack.pop()
return ''.join(stack).lstrip('0') or '0'
