11天【代码随想录算法训练营34期】 第五章 栈与队列part02(● 20. 有效的括号 ● 1047. 删除字符串中的所有相邻重复项 ● 150. 逆波兰表达式求值)

20. 有效的括号

class Solution:
    def isValid(self, s: str) -> bool:
        stk = []
        upper = ["(", "{", "["]
        lower = [")", "}", "]"]
        dictionary = {")":"(", "}":"{", "]":"["}

        for i in range(len(s)):
            if s[i] in upper:
                stk.append(s[i])
            elif s[i] in lower:
                if not stk or dictionary[s[i]] != stk[-1]:
                    return False
                else:
                    stk.pop()
        if stk:
            return False
        else:
            return True

1047. 删除字符串中的所有相邻重复项

class Solution:
    def removeDuplicates(self, s: str) -> str:
        stk = []
        for i in range(len(s)):
            if not stk:
                stk.append(s[i])
            elif stk[-1] == s[i]:
                stk.pop()
            else:
                stk.append(s[i])
        return ''.join(stk)

150. 逆波兰表达式求值

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stk = []
        operator = ['+', '-', '*', '/']

        for i in tokens:
            if i in operator and stk:
                temp_later = int(stk.pop())
                if not stk:
                    return 0
                temp_former = int(stk.pop())
                if i == '+':
                    temp_result = temp_former + temp_later
                elif i == '-':
                    temp_result = temp_former - temp_later
                elif i == '*':
                    temp_result = temp_former * temp_later
                elif i == '/':
                    temp_result = temp_former / temp_later
                stk.append(temp_result)
            else:
                stk.append(i)
        return int(stk[0])