0020-valid-parentheses

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Description

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.

 

Example 1:

Input: s = "()"

Output: true

Example 2:

Input: s = "()[]{}"

Output: true

Example 3:

Input: s = "(]"

Output: false

Example 4:

Input: s = "([])"

Output: true

Example 5:

Input: s = "([)]"

Output: false

 

Constraints:

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

Solution(Python)

class Solution:
    def isValid(self, s: str) -> bool:
        st = []
        mapping = {
            "(": ")",
            "[": "]",
            "{": "}",
        }
        for ch in s:
            if ch in "({[":
                st.append(ch)
            else:
                if not st:
                    return False 
                open_bck = st.pop()
                if  ch != mapping[open_bck]:
                    return False
        return True if not st else False

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