動機

複習stack

Problem

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.

 

Example 1:

Input: s = ()Output: true

Example 2:

Input: s = ()[]{}Output: true

Example 3:

Input: s = (]Output: false

Example 4:

Input: s = ([)]Output: false

Example 5:

Input: s = {[]}Output: true

 

Constraints:

  • 1 <= s.length <= 104
  • s consists of parentheses only '()[]{}'.

Sol

class Solution:
    def isValid(self, s: str) -> bool:
        stk = []
        tbl = {"(":")", "[":"]","{":"}"}
        lefts = list(tbl.keys())
        for c in s:
            if c in lefts:
                stk.append(c)
            elif len(stk) > 0 and tbl[stk[-1]] == c:
                stk.pop()
            else:
                return False
        return len(stk) == 0