Valid Parentheses

app.py

# 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.

### steps:
# Start with opening parentheses, the number of opening parentheses should equal closing parentheses
# we check if the closing parentheses matches the opening parentheses, then we remove it from consideration list
# when removing matched parentheses we remove from the last. use pop() function

class Solution:
  def isValid(self, s: str) -> bool:
    # define and empty stack
    stack = []
    
    ## create a hash map or dictionary that mapp all closing parentheses to the opening ones.
    closeToOpen = {")":"(", "]":"[", "{":"}"}
    
    ## loop through every character in the str pass to this function
    for i in s:
      # check is this charcater is a closing parentheses
      if i in closeToOpen:
        # if this's a close parentheses, let's make sure the stack is not empty and chack the value at the top is the matching opening parentheses
        # if they match pop the stack list
        
        if stack and stack[-1] == closeToOpen[i]:
          stack.pop()
        else:
          return False
       else:
        stack.append(c)
        
    return True is not stack else False