Symmetric Tree（中心对称树）

题目原型：

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

根据例子我们发现，每一层，给定两个指针分别从左端和右端开始，只有当左端的左子树的值等于右端的右子树的值，并且左端的右子树的值等于右端左子树的值时，此树便称为中心对称树（Symmetric Tree）。

	public boolean isSymmetric(TreeNode root)
	{
		if(root==null)
			return true;
		ArrayList
  
    list = new ArrayList
   
    (); list.add(root); boolean isFirst = true; while(list.size()>0) { if(list.size()%2==1&&isFirst==false) return false; for(int i = 0,j=list.size()-1;i<=j;i++,j--) { TreeNode one = list.get(i); TreeNode two = list.get(j); if(one.left!=null&&two.right!=null) { if(one.left.val!=two.right.val) return false; } if(one.right!=null&&two.left!=null) { if(one.right.val!=two.left.val) return false; } if((one.left==null&&two.right!=null)||(one.left!=null&&two.right==null)) return false; } //加入下一层节点 int size = list.size(); while(size>0) { if(list.get(0).left!=null) list.add(list.get(0).left); if(list.get(0).right!=null) list.add(list.get(0).right); list.remove(0); size--; } isFirst = false;//表示不是第一层了 } return true; }