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:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.

Example 1:

1 2	Input: s = "()" Output: true

Example 2:

1 2	Input: s = "()[]{}" Output: true

Example 3:

1 2	Input: s = "(]" Output: false

Example 4:

1 2	Input: s = "([)]" Output: false

Example 5:

1 2	Input: s = "{[]}" Output: true

定義符號對，遇到符號押入，配對成功移出，檢查陣列是否空。

Time: O(n), Space: O(n)

class Solution(object):
    def isValid(self, s):
        mapping = {
            "(": ")", 
            "[": "]", 
            "{": "}"}
        left = set(["(", "[", "{"])
        stack = []
        for i in s:
            if i in left: # match left
                stack.append(i) # append left
            elif stack and i == mapping[stack[-1]]: # stack not null and is right
                    stack.pop() # pop left
            else:
                return False
        return stack == [] # stack empty, all matched

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