Valid Parenthese

题目：

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.

Code:

#include <iostream>       // std::cout
#include <string>         // std::string
#include<stack>

using namespace std;

bool isValid (string const& s) {
    string left = "([{";
    string right = ")]}";
    stack<char> stk;
    for (auto c : s) {
        if (left.find(c) != string::npos) {
            stk.push (c);
        } else {
            if (stk.empty () || stk.top () != left[right.find (c)])
                return false;
            else
                stk.pop ();
        }
    }
    return stk.empty();
}

int main ()
{
  std::string str ("()[]()");
  
  bool flag = isValid(str);
  std::cout << flag << '\n';

  return 0;
}