20. 有效的括号

class Solution(object):def isValid(self, s):stack = []for x in s:if x == '(':stack.append(')')elif x == '{':stack.append('}')elif x == '[':stack.append(']')else:if not stack:return Falseelif stack:a = stack.pop()if x != a:return False          if stack:return Falsereturn True

394. 字符串解码

题目链接：394. 字符串解码

使用栈：遍历字符串，如果遇到的不是’]'，就把元素加到栈里，如果遇到']'，就将栈中’['之前的元素（即字符）提取出来作为要重复的元素，把'['弹出后，再把遇到的数字给提取出来，最终实现一次重复的操作。

class Solution(object):def decodeString(self, s):stack = []for i in s:if i != ']':stack.append(i)elif i == ']':strs = ''repeat = ''while stack[-1] != '[':strs = stack.pop() + strsstack.pop()  # 把'['弹出while stack and stack[-1].isdigit():repeat = stack.pop() + repeatstack.append(int(repeat) * strs)return ''.join(stack)