101. Symmetric Tree

/**
 * 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);
    }
    private boolean helper(TreeNode leftRoot, TreeNode rightRoot) {
        // if both of them are empty
        if (leftRoot == null && rightRoot == null) {
            return true;
        }
        // if one of them is not empty
        if (leftRoot == null || rightRoot == null) {
            return false;
        }
        // if they are not equal
        if (leftRoot.val != rightRoot.val) {
            return false;
        }
        return helper(leftRoot.left, rightRoot.right) && helper(leftRoot.right, rightRoot.left);
    }
}

/**
 * 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> queue = new LinkedList<>();
        queue.addLast(root.left);
        queue.addLast(root.right);
        while (!queue.isEmpty()) {
            TreeNode leftRoot = queue.removeFirst();
            TreeNode rightRoot = queue.removeFirst();
            // if both of them are empty, ignore the rest code of this round
            if (leftRoot == null && rightRoot == null) {
                continue;
            }
            // if one of the sub-tree is not empty
            if (leftRoot == null || rightRoot == null) {
                return false;
            }
            // if they are not equal
            if (leftRoot.val != rightRoot.val) {
                return false;
            }
            queue.addLast(leftRoot.left);
            queue.addLast(rightRoot.right);
            queue.addLast(rightRoot.left);
            queue.addLast(leftRoot.right);
        }
        return true;
    }
}