Leetcode -- 20. Valid Parentheses

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.

bool isValid(string s) {
stack<char> str;
for (int i = 0; i < s.length(); i++)
{
if (s[i] == '(' || s[i] == '[' || s[i] == '{')
{
str.push(s[i]);
}
else
{
if (str.empty())
return false;
if (str.top() == '(' && s[i] == ')' || str.top() == '[' && s[i] == ']' || str.top() == '{' && s[i] == '}')
{
str.pop();
}
else
{
return false;
}
}
}
if (str.empty())
return true;
else
return false;
}