20. Valid Parentheses

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.
3. Every close bracket has a corresponding open bracket of the same type.

\

Solution:

• 对于每个闭括号，我们想要找到一个对应的开括号
• 我们可以维护一个stack，只往stack里加入开括号
• 当遇到闭括号时，我们判断对应的开括号是不是位于stack的-1位，如果是的话我们把开括号pop出来；如果对不上/stack为空，我们返回False
• 在遍历s结束后，我们看stack是否为空，因为每一对括号都能对应上的话，stack最后应该是空

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

        for c in s:
            if c in cto:
                if stack and stack[-1] == cto[c]:
                    stack.pop()
                else:
                    return False
            else:
                stack.append(c)
        
        return not stack

155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.