括號匹配問題

題目

Imagine you are building a compiler. Before running any code, the compiler must check that the parentheses in the program are balanced. Every opening bracket must have a corresponding closing bracket. We can approximate this using strings.

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:

• Open brackets are closed by the same type of brackets.
• Open brackets are closed in the correct order.
• Note that an empty string is also considered valid.

Example:
Input: "((()))"
Output: True

Input: "[()]{}"
Output: True

Input: "({[)]"
Output: False

分析

建立一個棧。
而後開始從左到右枚舉每一個字符，
遇到左括號則進棧；
遇到右括號，則出棧棧頂的元素。判斷該元素是否和當前的右括號匹配：
如不匹配，則返回 false.
如匹配，則繼續下一個循環；
判斷棧是否爲空。爲空則返回 true, 不然返回 false.

時間複雜度爲 O(n).

代碼

class Solution:
  def isValid(self, s):
    stack = []
    matches = {
        "}": "{",
        "]": "[",
        ")": "("
    }
    for c in s:
        if c in matches.values(): # 左括號
            stack.append(c)
        elif c in matches.keys(): # 右括號
            left = stack.pop()
            if left == matches[c]:
                continue
            else:
                return False
        else:
            continue
    
    return len(stack) == 0


# Test Program
s = "()(){(())" 
# should return False
print(Solution().isValid(s))

s = ""
# should return True
print(Solution().isValid(s))

s = "([{}])()"
# should return True
print(Solution().isValid(s))