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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容易理解，此题位简单堆栈问题，将信息推入栈中，右括号出现时pop判断是否符合标准即可，代码如下：

class Solution {public:    int isValid(string s) {        int i=0;        vector<int> v;        for(i=0;i<s.size();i++){        if(s[i]=='(')  v.push_back(1);        if(s[i]=='[')  v.push_back(2);        if(s[i]=='{')  v.push_back(3);              if(s[i]==')'){              if(v.empty()) return false;              if(v[v.size()-1]==1) v.pop_back();              else return false;              }              if(s[i]==']'){              if(v.empty()) return false;              if(v[v.size()-1]==2) v.pop_back();              else return false;              }              if(s[i]=='}'){              if(v.empty()) return false;              if(v[v.size()-1]==3) v.pop_back();              else return false;              }        }        if(!v.empty()) return false;        return true;    }};