25期代码随想录算法训练营第十天 | 栈与队列 part 2

20. 有效的括号

链接

class Solution:
    def isValid(self, s: str) -> bool:
        closeToOpen = {")": "(", "]":"[", "}":"{"}
        stack = []

        for string in s:
            if string not in closeToOpen:
                stack.append(string)
            else:
                if not stack or stack.pop() != closeToOpen[string]:
                    return False
        
        return len(stack) == 0
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1047. 删除字符串中的所有相邻重复项

链接

class Solution:
    def removeDuplicates(self, s: str) -> str:
        stack = []

        for string in s:
            if not stack:
                stack.append(string)
            else:
                if string == stack[-1]:
                    stack.pop()
                else:
                    stack.append(string)
        return ''.join(stack)
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150. 逆波兰表达式求值

链接

from operator import add, mul, sub
class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        op_map = {"+": add, "-": sub, "*": mul, "/": lambda x,y: int(x/y)}

        for token in tokens:
            if token not in op_map:
                stack.append(token)
            else:
                y = int(stack.pop())
                x = int(stack.pop())
                res = op_map[token](x, y)
                stack.append(res)
        
        return int(stack[-1])
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