Algorithm-LeetCode/Solutions/0020.Valid_Parentheses.py at main · ling67/Algorithm-LeetCode · GitHub

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"""
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:

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

Input: s = "()[]{}"
Output: true
Example 3:

Input: s = "(]"
Output: false


Constraints:

1 <= s.length <= 104
s consists of parentheses only '()[]{}'.
"""

class Solution:
    def isValid(self, s: str) -> bool:
        if len(s) % 2 != 0:
            return False

        mapping = {"(":")", "[":"]", "{":"}"}

        st = []
        for ch in s:
            if ch in "([{":
                st.append(ch)
            else:
                if len(st) == 0 or mapping[st[-1]] != ch:
                    return False
                st.pop()
        return len(st) == 0