[2018-Interview] Symmetric Tree

Original question: https://leetcode.com/problems/symmetric-tree/description/

My answer:

Recursive:

/**
 * 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;
        }
        return helper(root.left, root.right);
    }
    
    boolean helper(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }

        if ((left == null && right != null) ||
            (left != null && right == null)
           ) {
            return false;
        } else {
            // left != null && right != null
            if (left.val == right.val) {
                return helper(left.left, right.right) && helper(left.right, right.left);   
            } else {
                return false;
            }
        }
    }
}

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) {
        if (root == null) {
            return true;
        }
        LinkedList<TreeNode> currentLevel = new LinkedList<TreeNode>();
        LinkedList<TreeNode> nextLevel = new LinkedList<TreeNode>();
        currentLevel.addLast(root);
        while (true) {
            while(!currentLevel.isEmpty()) {
                TreeNode temp = currentLevel.removeFirst();
                if (temp != null) {
                    nextLevel.addLast(temp.left);
                    nextLevel.addLast(temp.right);
                }
            }
            if (nextLevel.isEmpty()) {
                break;
            }
            int left = (nextLevel.size() - 1) / 2 ;
            int right = left + 1;
            while (left >= 0 && right < nextLevel.size()) {
                if (!equals(nextLevel.get(left), nextLevel.get(right))) {
                    return false;
                }
                left --;
                right ++;
            }
            currentLevel.addAll(nextLevel);
            nextLevel = new LinkedList<TreeNode>();
        }
        return true;
    }

    boolean equals(TreeNode node1, TreeNode node2) {
        if (node1 == null && node2 == null) {
            return true;
        } else {
            if (node1 != null && node2 != null && node1.val == node2.val) {
                return true;
            }
        }
        return false;
    }
}