20. Valid Parentheses.py

# Given a string 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.
# Note that an empty string is also considered valid.

# Example 1:

# Input: "()"
# Output: true
# Example 2:

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

# Input: "(]"
# Output: false
# Example 4:

# Input: "([)]"
# Output: false
# Example 5:

# Input: "{[]}"
# Output: true

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        p = ''
        for x in s:
            if x != ')' and x != '}' and x != ']':
                stack.append(x)
            else:
                if len(stack) > 0:
                    p = stack.pop()
                else:
                    return False
                if ( x == ')' and p == '(') or ( x == '}' and p == '{') or ( x == ']' and p == '['):
                    continue
                else:
                    return False
        if len(stack) > 0:
            return False
        else:
            return True