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
题目含义：给定一个树，判断左右子树是否完全对称

    private boolean isMirror(TreeNode p,TreeNode q)
    {
        if (p==null && q==null) return true;
        if(p==null || q==null || p.val!=q.val) return false;
        return isMirror(p.left,q.right) && isMirror(p.right,q.left);
    } 
    
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isMirror(root.left,root.right);        
    }