class Solution {
    public boolean isValid(String s) {
        Deque<Character> deque = new LinkedList<>();
        char ch;
        for (int i = 0; i<s.length();i++){
            ch = s.charAt(i);
            if (ch == '('){
                deque.push(')');
            } else if (ch == '{') {
                deque.push('}');
            } else if (ch == '[') {
                deque.push(']');
            } else if (deque.isEmpty() || deque.peek() != ch) {
                return false;
            } else {
                deque.pop();
            }
        }
        return deque.isEmpty();
    }
}

class Solution {
    public String removeDuplicates(String s) {
        Deque<Character> deque = new LinkedList<>();
        char ch;
        for (int i = 0; i < s.length();i++) {
            ch = s.charAt(i);
            if (deque.isEmpty()||deque.peek() != ch) {
                deque.push(ch);
            } else {
                deque.pop();
            }
        }
        String str = "";
        while (!deque.isEmpty()) {
            str = deque.pop() + str;
        }
        return str;
    }
}

class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> stack = new LinkedList();
        for (String s : tokens) {
            if ("+".equals(s)) {
                stack.push(stack.pop() + stack.pop());    
            } else if ("-".equals(s)) {
                stack.push(-stack.pop() + stack.pop());
            } else if ("*".equals(s)) {
                stack.push(stack.pop() * stack.pop());
            } else if ("/".equals(s)) {
                int temp1 = stack.pop();
                int temp2 = stack.pop();
                stack.push(temp2 / temp1);
            } else {
                stack.push(Integer.valueOf(s));
            }
        }
        return stack.pop();
    }
}