【bilibili】有效的括号

class Solution {
    public boolean isValid(String s) {
        if (s.length() == 0 || s == null) {
      return true;
    }
        if(s.length() % 2 == 1){
            return false;
        }
    Stack<Character> stack = new Stack<Character>();
    for (int i = 0; i < s.length(); i++) {
      if (s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') {
        stack.add(s.charAt(i));
      } else if (s.charAt(i) == ')') {
        if (stack.size() == 0 || stack.pop() != '(') {
          return false;
        }
      } else if (s.charAt(i) == ']') {
        if (stack.size() == 0 || stack.pop() != '[') {
          return false;
        }
      } else {
        if (stack.size() == 0 || stack.pop() != '{') {
          return false;
        }
      }
    }
    if (stack.size() != 0) {
      return false;
    } else {
      return true;
    }
    }
}