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.

class Solution {
public:
    bool isValid(string s) {
    std::vector<char> ans;
	for (int i = 0; i < s.size(); i++)
	{
		switch (s[i])
		{
		case ‘(‘: ans.push_back(s[i]); break;
		case ‘[‘: ans.push_back(s[i]); break;
		case ‘{‘: ans.push_back(s[i]); break;
		case ‘)‘: if (ans.size() == 0 || ans[ans.size() - 1] != ‘(‘)
				  {
					  return false;
				  }
				  ans.pop_back();
				  break;
		case ‘]‘: if (ans.size() == 0 || ans[ans.size() - 1] != ‘[‘)
				  {
					  return false;
				  }
				  ans.pop_back();
				  break;
		case ‘}‘: if (ans.size() == 0 || ans[ans.size() - 1] != ‘{‘)
				  {
					  return false;
				  }
				  ans.pop_back();
				  break;
		}
		
	}
	return ans.empty();
    }
};