LeetCode 20. Valid Parentheses

[Woodyiiiiiii]

Given a string 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.
   Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

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

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

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题目要求匹配括号，很简单。C++解法如下：

class Solution {
public:
    bool isValid(string s) {
        stack<char> help;
        for (int i = 0; i < s.size(); ++i) {
            char c = s[i];
            if (c == '(' || c == '[' || c == '{')
                help.push(c);
            else {
                if (help.empty()) return false;
                char old = help.top();
                help.pop();
                if (c == ')' && old != '(')
                    return false;
                else if (c == ']' && old != '[')
                    return false;
                else if (c == '}' && old != '{')
                    return false;
            }
        }
        if (!help.empty()) return false;
        else return true;
    }
};

Java的栈(容器)是不能存储基本类型的，只能做一个标记:

class Solution {
    public boolean isValid(String s) {
        if (s.isEmpty()) {
            return true;
        }
        Stack<Integer> help = new Stack();
        int i = 0;
        for (; i < s.length(); ++i) {
            if (s.charAt(i) == '(')
                help.push(1);
            else if (s.charAt(i) == '[')
                help.push(2);
            else if (s.charAt(i) == '{')
                help.push(3);
            else {
                char now = s.charAt(i);
                if (help.empty())
                    return false;
                int old = help.pop();
                if (now == ')' && old != 1)
                    return false;
                else if (now == ']' && old != 2)
                    return false;
                else if (now == '}' && old != 3)
                    return false;
            }
        }
        if (help.empty())
            return true;
        else
            return false;
    }
}

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参考资料:
LeetCode原题