20. Valid Parentheses

1.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.

2.分析

3.代码

class Solution {
public:
    bool isValid(string s) {
        stack st;
        for (unsigned int i = 0; i < s.size(); ++i) {
            if ('(' == s[i] || '{' == s[i] || '[' == s[i]) {
                st.push(s[i]);
            } else {
                if (st.empty()) return false;
                char ch = st.top();
                switch (s[i]) {
                    case ')': {
                        if (ch != '(') return false; 
                        st.pop();
                        break;
                    }
                    case '}': {
                        if (ch != '{') return false; 
                        st.pop();
                        break;
                    }
                    case ']': {
                        if (ch != '[') return false;
                        st.pop();
                        break;
                    }
                    default: return false;                    
                }
            }
        }
        return st.empty() ? true : false;
    }
};