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.

典型的栈的应用，直接附上代码：

C++ code
    bool isValid(string s) {
        stack<char> parentheses;
        for (auto c : s) {
            if (c == ‘(‘ || c == ‘{‘ || c == ‘[‘)
                parentheses.push(c);
            else {
                if (parentheses.empty())
                    return false;
                else if (c == ‘)‘ && parentheses.top() != ‘(‘)
                    return false;
                else if (c == ‘]‘ && parentheses.top() != ‘[‘)
                    return false;
                else if (c == ‘}‘ && parentheses.top() != ‘{‘)
                    return false;

                parentheses.pop();
            }
        }

        return parentheses.empty();
    }

LeetCode[stack]: Valid Parentheses