给定一个只包括'(',')','{','}','[',']'
的字符串,判断字符串是否有效。
有效字符串需满足:
左括号必须用相同类型的右括号闭合。
左括号必须以正确的顺序闭合。
注意空字符串可被认为是有效字符串。
示例 1:
输入: "()"
输出: true
示例 2:
输入: "()[]{}"
输出: true
示例 3:
输入: "(]"
输出: false
示例 4:
输入: "([)]"
输出: false
示例 5:
输入: "{[]}"
输出: true
class Solution(object):
def isValid(self, s):
"""
:type s: str
:rtype: bool
"""
# The stack to keep track of opening brackets.
stack = []
# Hash map for keeping track of mappings. This keeps the code very clean.
# Also makes adding more types of parenthesis easier
mapping = {")": "(", "}": "{", "]": "["}
# For every bracket in the expression.
for char in s:
# If the character is an closing bracket
if char in mapping:
# Pop the topmost element from the stack, if it is non empty
# Otherwise assign a dummy value of '#' to the top_element variable
top_element = stack.pop() if stack else '#'
# The mapping for the opening bracket in our hash and the top
# element of the stack don't match, return False
if mapping[char] != top_element:
return False
else:
# We have an opening bracket, simply push it onto the stack.
stack.append(char)
# In the end, if the stack is empty, then we have a valid expression.
# The stack won't be empty for cases like ((()
return not stack