101. Symmetric Tree

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
    1
   /   2   2
 / \ / 3  4 4  3

But the following [1,2,2,null,3,null,3] is not:?
    1
   /   2   2
   \      3    3



/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
      if(root == null){
        return true;
      }

      TreeNode left = root.left;
      TreeNode right = root.right;
      return helper(left, right);
        
    }
    private boolean helper(TreeNode left, TreeNode right){
      if(left == null && right == null){
        return true;
      }else if( left == null || right == null){
        return false;
      }else{
        if( left.val != right.val){
          return false;
        }else{
          return helper(left.right, right.left) && helper(left.left, right.right);
        }
      }
    }
}






// iterative 
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
      Queue<TreeNode> queue = new LinkedList<>();
      if(root == null){
        return true;
      }
      queue.offer(root.left);
      queue.offer(root.right);
      while(!queue.isEmpty()){
        TreeNode left = queue.poll();
        TreeNode right = queue.poll();
        
        if(left == null && right == null) continue; // continue instead of true, because there are more nodes in the queue waiting to be checked 
        else if(left == null || right == null || left.val != right.val) return false;
        else{
          queue.offer(left.left);
          queue.offer(right.right);
          queue.offer(left.right);
          queue.offer(right.left);
        }
      }
      return true;
    }
}

// add and poll can be used together with no problem??? 
// cant use ArrayDeque to implement this queue , otherwise error? why. Because in arraydeque, when you add or offer an null into the queue. It gives you NPE. In linked list its okay to add or offer null.