20. Valid Parentheses

题目

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.

解析

class Solution_20 {
public:
    bool isValid(string s) {
        if (s.size()<1)
        {
            return true;
        }
        stack<char> sta;
        for (int i = 0; i < s.size();i++)
        {
            if (s[i]=='('||s[i]=='['||s[i]=='{')
            {
                sta.push(s[i]); //入栈
            }else if ((s[i] == ')' || s[i] == ']' || s[i] == '}')&&sta.empty())
            {
                return false;
            }
            else if ((s[i] == ')'&&sta.top() == '(') || (s[i] == ']'&&sta.top() == '[') || (s[i] == '}'&&sta.top() == '{'))
            {
                sta.pop();
            }else 
            {
                return false;
            }
        }

        if (sta.empty())
        {
            return true;
        }
        else
        {
            return false;
        }
    }
};

• 20. Valid Parentheses