Valid Parentheses.txt

problem:
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.
------------------------------------------------------------------------
solution:
//用栈作辅助来模拟这个过程
//注意简化的部分，比如s.length()为0或者为奇数个时肯定不满足条件，还有当当前字符不在指定字符范围内时也可直接返回false

char symCharacter(char c)
{
  switch(c)
	{
		case '(':
			return ')';
		case '[':
			return ']';
		case '{':
			return '}';
		default:     //用返回'0'来标记当前字符无效
			return '0';
	}
}
bool isValid(string s)
{
	stack<char> pstack;
	if(s.length() == 0 || s.length() % 2 != 0)
		return false;
	for(int i = 0; i < s.length(); i++)
	{
		if(pstack.empty())
			pstack.push(s[i]);
		else
		{
			char c = pstack.top();
			char t = symCharacter(c);
			if(t== s[i])
				pstack.pop();
			else if(t == '0')
				return false;
			else
				pstack.push(s[i]);
		}
	}
	if(pstack.empty())
		return true;
	else
		return false;
}