1.题目详情

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

2.代码详情

python：

class Solution(object):def isValid(self, s):stack = []mapping = {")": "(", "}": "{", "]": "["}for char in s:if char in mapping:top_element = stack.pop() if stack else '#'if mapping[char] != top_element:return Falseelse:stack.append(char)return not stack

java：

class Solution {private HashMap<Character, Character> mappings;public Solution() {this.mappings = new HashMap<Character, Character>();this.mappings.put(')', '(');this.mappings.put('}', '{');this.mappings.put(']', '[');}public boolean isValid(String s) {Stack<Character> stack = new Stack<Character>();for (int i = 0; i < s.length(); i++) {char c = s.charAt(i);if (this.mappings.containsKey(c)) {char topElement = stack.empty() ? '#' : stack.pop();if (topElement != this.mappings.get(c)) {return false;}} else {stack.push(c);}}return stack.isEmpty();}
}