Valid Parentheses

问题：The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.，判断符合条件的符号（[]）也符合
分析：遇到左边符号进栈，右边符号就将栈顶出栈，若和当前遍历的符号相对应则继续遍历下一个符号，若不对应返回false

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class Solution { public:     bool isValid(string s) {         stack<char> S;         char st;         for(int i=0;i<s.size();i++)         {             if(s[i]=='(' || s[i]=='[' || s[i]=='{') S.push(s[i]);             else if(S.empty()) return false;//如果栈为空出栈会导致RE             else if(s[i]==')')             {                 st=S.top();                 if(st=='(') S.pop();                 else return false;             }             else if(s[i]==']')             {                 st=S.top();                 if(st=='[') S.pop();                 else return false;             }             else if(s[i]=='}')             {                 st=S.top();                 if(st=='{') S.pop();                 else return false;             }         }         if(!S.empty()) return false;         return true;     } };