Valid Parentheses -- LeetCode

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.

思路：用栈。要注意的一点是，当读到了右括号时，在判断栈顶是否是对应的左括号前，要先确定栈是否为空。

 1 class Solution {
 2 public:
 3     bool isValid(string s) {
 4         stack<char> va;
 5         for (auto i : s)
 6         {
 7             if (i == '(' || i == '{' || i == '[')
 8                 va.push(i);
 9             else if (va.empty()) return false;
10             else if (i == ')' && va.top() != '(')
11                 return false;
12             else if (i == '}' && va.top() != '{')
13                 return false;
14             else if (i == ']' && va.top() != '[')
15                 return false;
16             else va.pop();
17         }
18         return va.empty();
19     }
20 };