Valid Parentheses -- leetcode

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.

class Solution {
public:
    bool isValid(string s) {
        stack<char, vector<char> > lifo;
        lifo.push('#');

        for (size_t i=0; i<s.size(); i++) {
                if (s[i] == '{' ||
                    s[i] == '[' ||
                    s[i] == '(')
                        lifo.push(s[i]);
                else {
                        const char ch = lifo.top();
                        lifo.pop();
                        if ((s[i] == '}' && ch != '{') ||
                            (s[i] == ']' && ch != '[') ||
                            (s[i] == ')' && ch != '('))
                                return false;
                }
        }

        lifo.pop();
        return lifo.empty();
    }
};

提前进栈一个字符，省得循环中检查栈空了。

在leetcode上的运行时间为4ms。