Description： 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

題意：判斷一顆二叉樹是否為一顆映象二叉樹

解法：映象二叉樹如圖所示，其左子樹的所有節點翻轉後與右子樹對應節點相同；因此，我們只需要遞迴的計算二叉樹的根節點與其左右兩個子樹的節點，相對應翻轉後是否相同；

Java

/**
 * 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) {
        return isMirror(root, root);
    }
    private boolean isMirror(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) return true;
        if (root1 == null || root2 == null) return false;
        if (root1.val != root2.val) return false;
        return isMirror(root1.left, root2.right) && isMirror(root1.right, root2.left);
    }
    
}