Solution to Leetcode 20 Valid Parentheses

soln_lc20.py

class Solution:
    def isValid(self, s: str) -> bool:
        dic = {"(":")", "{":"}", "[":"]"}
        stack = []
        # we know all char in s is either key or value in dic
        for i in s:
            # if key, store in stack
            if i in dic:
                stack.append(i)
            # if value, check whether there is corresponding key
            elif len(stack)== 0 or dic[stack.pop()] != i:
                return False 
        # e.g., when s = "["
        return not stack