[LeetCode] Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3
But the following is not:

    1
   / \
  2   2
   \   \
   3    3
Note:
Bonus points if you could solve it both recursively and iteratively.

[Solution].java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root == null) return true;
        return isSymmetric(root.left, root.right);
    }

    public boolean isSymmetric(TreeNode a, TreeNode b){
        if(a == null && b == null) return true;
        else if(a == null || b == null) return false;
        else if(a.val != b.val) return false;
        return isSymmetric(a.left, b.right) && isSymmetric(a.right, b.left);
    }
}