reversell.cpp

Node* Reverse(Node *head)
{
    Node* trail=0;
    while (head){
        Node* next= head->next;
        head->next = trail;
        trail=head;
        head=next;
    }
    return trail;
}

validparens.cpp

bool isValid(string s) {
	stack<char> st;
	for (int i = 0; i < s.length(); i++) {
		if (s[i] == '(' || s[i] == '[' || s[i] == '{')
			st.push(s[i]);
		else  {
			if (!st.empty()) {
				switch (st.top())
							{
							case '(':
								if (s[i] == ')') st.pop();
								else return false;
								break;
							case '{':
								if (s[i] == '}') st.pop();
								else return false;
								break;
							case '[':
								if (s[i] == ']') st.pop();
								else return false;
								break;
							}
			}
			else return false;
			
		}
	}
	return st.empty();
}