402. 移掉 K 位数字

单调栈

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        stack = []
        remove_num = 0
        for i in range(len(num)):
            while stack and num[i] < stack[-1] and k > remove_num:
                stack.pop()
                remove_num = remove_num + 1
            stack.append(num[i])
        
        if k > remove_num:
            stack = stack[:len(stack)+remove_num-k]
        
        while stack and stack[0] == '0':
            stack = stack[1:]
        if not stack:
            return '0'
        else:
            return ''.join(stack)