101. Symmetric Tree

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

Note:Bonus points if you could solve it both recursively and iteratively.

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        return root != null? Symmetric(root.left,root.right):true;
    }
    private boolean Symmetric(TreeNode L,TreeNode R)
    {
        if(L==null&&R==null)
           return true;
        if(L==null||R==null)  // 已经将都是null相等的情况排除了
           return false;
        return L.val == R.val&&Symmetric(L.left,R.right)&&Symmetric(L.right,R.left);
    }
}