20. Valid Parentheses

自認寫的有點鳥... 再想想看

題目原文

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

解題思路

1. 善用stack。

2. 遇到左括弧先push到Stack裡，遇到右括弧再從Stack pop出左括弧。

3. pop出來的左括弧是否對應相對的右括弧。

4. 最後，判斷Stack是否為空。

程式解答

class Solution 
{
public:
    bool isValid(string s) 
    {
        std::stack<char> p;
        char temp;
        
        if (s.length() % 2 != 0)
            return false;
        
        for (int i = 0; i < s.length(); i++)
        {
            switch (s[i])
            {
                case '(':
                case '[':
                case '{':
                    p.push(s[i]);
                    break;
                case ')':
                    if (!p.size())
                        return false;
                    temp = p.top();
                    p.pop();
                    if (temp != '(')
                        return false;
                    break;
                case ']':
                    if (!p.size())
                        return false;
                    temp = p.top();
                    p.pop();
                    if (temp != '[')
                        return false;
                    break;
                case '}':
                    if (!p.size())
                        return false;
                    temp = p.top();
                    p.pop();
                    if (temp != '{')
                        return false;
                    break;
            }
        }
        if (!p.size())
            return true;
        return false;
    }
};