20. 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.

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设置一个栈，保存左边的括号，与右边的括号匹配对应。

class Solution {
public:
    bool isValid(string s) {
        int len=s.size();
        stack<char>st;
        for(int i=0;i<len;i++){
            if(s[i]=='('||s[i]=='{'||s[i]=='[')
                st.push(s[i]);
            else {
                if(st.empty())
                    return false;
                else {
                    int t=st.top();
                    if ((t == '(' && s[i] != ')') || (t == '[' && s[i] != ']') || (t == '{' && s[i] != '}'))
                         return false;
                    st.pop();
                }
            }
        }
        return st.empty();
    }
};