Valid Parentheses

题目描述：

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

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

　　典型的用stack解决的问题。

solution：

bool isValid(string s) {
    if (s.empty() || s.size() % 2 != 0)
        return false;
    stack<char> stk;
    for (int i = 0;i < s.size();++i)
    {
        if(s[i] == '(' || s[i] == '[' || s[i] == '{')
            stk.push(s[i]);
        else
        {
            if (stk.empty())
                return false;
            char ch = stk.top();
            stk.pop();
            if(s[i] == ')' && ch != '(')
                return false;
            if (s[i] == ']' && ch != '[')
                return false;
            if (s[i] == '}' && ch != '{')
                return false;
        }
    }
    return stk.empty();
}

　　如有其它解法，欢迎来稿。