20. Valid Paranthesis

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

    def isValid(self, s: str) -> bool:
        symlis = []
        
        for ch in s:
            if ch == '{' or ch == '(' or ch=='[':
                symlis.append(ch)
            elif ch == '}' and symlis:
                if symlis[-1]=='{':
                    symlis.pop()
                else:
                    return False
            elif ch == ']' and symlis:
                if symlis[-1]=='[':
                    symlis.pop()
                else:
                    return False
            elif ch == ')' and symlis:
                if symlis[-1]=='(':
                    symlis.pop()
                else:
                    return False
            else:
                return False
                
        if len(symlis)==0:
            return True

def isValid(s):
    stack = []
    bracket_pairs = {')': '(', '}': '{', ']': '['}

    for char in s:
        if char in bracket_pairs.values():
            stack.append(char)
        elif char in bracket_pairs.keys():
            if not stack or bracket_pairs[char] != stack.pop():
                return False
        else:
            return False

    return not stack