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):
         """
         :type s: str
         :rtype: bool
         """
         if len(s)%2 != 0:
             return(False)
         
         stack=[]
         for i in range(len(s)):
             stack.append(s[i])
             if stack[0] == ')' or stack[0] == ']' or stack[0] == '}':
                 return(False)
             if stack[-1] == ')' :
                 if stack[-2] =='(':
                     stack.pop()
                     stack.pop()
             elif stack[-1] == ']' :
                 if stack[-2] =='[':
                     stack.pop()
                     stack.pop()
             elif stack[-1] == '}':
                 if stack[-2] =='{':
                     stack.pop()
                     stack.pop()
         if stack == []:
             return(True)
         else:
             return(False)
class Solution:
     def isValid(self, s):
         """
         :type s: str
         :rtype: bool
         """
         if len(s)%2 != 0:
             return(False)
         
         stack=[]
         for i in range(len(s)):
             stack.append(s[i])
             if stack[0] == ')' or stack[0] == ']' or stack[0] == '}':
                 return(False)
             if stack[-1] == ')' :
                 if stack[-2] =='(':
                     stack.pop()
                     stack.pop()
             elif stack[-1] == ']' :
                 if stack[-2] =='[':
                     stack.pop()
                     stack.pop()
             elif stack[-1] == '}':
                 if stack[-2] =='{':
                     stack.pop()
                     stack.pop()
         if stack == []:
             return(True)
         else:
             return(False)