栈与队列20|1047|150

20有效的括号

class Solution:
    def isValid(self, s: str) -> bool:
        s_list = list(s)
        if len(s_list) % 2 != 0:
            return False
        stack = []
        for item in s_list:
            if item == "(":
                stack.append(")")
            elif item == "[":
                stack.append("]")
            elif item == "{":
                stack.append("}")
            elif item == ")" or item == "]" or item == "}":
                if len(stack) == 0 or stack[-1] != item:
                    return False
                elif stack[-1] == item:
                    stack.pop()
        if len(stack) == 0:
            return True
        elif len(stack) != 0:
            return False

1047删除字符串中的所有相邻重复项

class Solution:
    def removeDuplicates(self, s: str) -> str:
        stack = []
        s_list = list(s)
        result = ""
        for item in s_list:
            if len(stack) == 0 or stack[-1] != item:
                stack.append(item)
            elif stack[-1] == item:
                stack.pop()
        for item in stack:
            result += item
        return result

150逆波兰表达式求值

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        for item in tokens:
            if item in {"+", "-", "*", "/"}:
                num2 = stack.pop()
                num1 = stack.pop()
                if item == "+":
                    val = num1 + num2
                elif item == "-":
                    val = num1 - num2
                    
                elif item == "*":
                    val = num1 * num2
                elif item == "/":
                    val = num1 // num2
                    if val < 0 and num1 % num2 != 0:
                        val += 1
                stack.append(val)
            else:
                stack.append(int(item))
        return stack.pop()