Java 括号序列

 public boolean isValid (String s) {
     // write code here
             if (s.length() % 2 != 0) return false;
     Stack<Character> stack = new Stack<>();
     for (int i = 0; i < s.length(); i++) {
         if (stack.empty()) {
             stack.add(s.charAt(i));
         } else if (s.charAt(i) == stack.peek()) {
             stack.add(s.charAt(i));
         } else {
             if (s.charAt(i) == ')' && stack.peek() == '(') {
                 stack.pop();
             } else if (s.charAt(i) == '}' && stack.peek() == '{') {
                 stack.pop();
             } else if (s.charAt(i) == ']' && stack.peek() == '[') {
                 stack.pop();
             }
         }
     }
     if (stack.empty()) return true;
     else return false;