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
題目含義：給定一個樹，判斷左右子樹是否完全對稱

 1
 
     private boolean isMirror(TreeNode p,TreeNode q)
 2     {
 3         if (p==null && q==null) return true;
 4         if(p==null || q==null || p.val!=q.val) return false;
 5         return isMirror(p.left,q.right) && isMirror(p.right,q.left);
 6     } 
 7     
 8     public boolean
 
 isSymmetric(TreeNode root) {
 9         if (root == null) return true;
10         return isMirror(root.left,root.right);        
11     }

101. Symmetric Tree