private static boolean validParentheses(String s) {
Stack<Character> stack = new Stack<Character>();
for (int i = 0; i < s.length(); i++) {
char cur = s.charAt(i);
if (cur == '(' || cur == '[' || cur == '{') { // Left parentheses, push into stack.
stack.push(cur);
} else { // Right parentheses, compare with top element of stack.
if (stack.isEmpty())
return false; // If stack is empty and current character is right parentheses, it is
// invalid.
if (samePair(stack.peek(), cur)) {
stack.pop();
} else {
return false;
}
}
}
if (stack.isEmpty()) return true;
else return false;
}
private static int evalRPN(String[] tokens) {
Stack<String> stack = new Stack<String>();
for (int i = 0; i < tokens.length; i++) {
String cur = tokens[i];
switch (cur.charAt(0)) {
case '+':
stack.push(String.valueOf(Integer.parseInt(stack.pop()) + Integer.parseInt(stack.pop())));
break;
case '-':
int num2 = Integer.parseInt(stack.pop());
int num1 = Integer.parseInt(stack.pop());
stack.push(String.valueOf(num1 - num2));
break;
case '*':
stack.push(String.valueOf(Integer.parseInt(stack.pop()) * Integer.parseInt(stack.pop())));
break;
case '/':
int divisor = Integer.parseInt(stack.pop());
int dividend = Integer.parseInt(stack.pop());
stack.push(String.valueOf((Integer) (dividend / divisor)));
break;
default:
stack.push(cur);
break;
}
}
return Integer.parseInt(stack.pop());
}