LeetCode OJ - Valid Parentheses

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) {
        int len = s.size();
        if(len == 0) return true;
        
        stack<char> st;
        for(int i = 0; i < len; i++) {
            //入栈
            if(s[i] == '(' || s[i] == '{' || s[i] == '[') {
                st.push(s[i]);
            } else {
            //出栈
                if(st.empty()) return false;
                char left = st.top();
                st.pop();
                if(left == '(' && s[i] != ')') {
                    return false;
                } else if(left == '{' && s[i] != '}') {
                    return false;
                } else if(left == '[' && s[i] != ']') {
                    return false;   
                }
            }
        }
        
        if(!st.empty()) return false;
        return true;
    }
};