LeetCode101 SynmetricTree Java题解

题目：

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

解题：

是否对称就是要求每一层从左到右刚好等于从右到左，有两种解法一种非递归，一种递归。

非递归：

public static boolean isSymmetric(TreeNode root) {

		if(root==null)
			return true;
		Queue<TreeNode> left=new LinkedList<>();
		Queue<TreeNode> right=new LinkedList<>();
		if(root.left==null)
			left.offer(root.left);
		if(root.right==null)
			right.offer(root.right);
		while(!left.isEmpty()&&!right.isEmpty())
		{
			TreeNode l=left.poll();
			TreeNode r=right.poll();
			if(l==null&&r==null) continue;
			if(l==null||r==null) return false;
			if(l.val!=r.val) return false;
			left.offer(l.left);left.offer(l.right);
			right.offer(r.right);right.offer(r.left);//倒序插入 形成镜像

		}
		if(left.isEmpty()&&right.isEmpty())
			return true;
		else {
			return false;
		}

    }
	

递归：

 public boolean isSymmetric2(TreeNode root) {
		 if(root==null)
			 return true;
		 else {
			return  isSame(root.left, root.right);
		}

	 }

	 public boolean isSame(TreeNode left,TreeNode right)
	 {
		 if(left==null&& right==null)
			 return true;
		 if(left==null||right==null)
			 return false;
		 if(left.val!=right.val)
			 return false;
		 else {
			return isSame(left.left, right.right)&&isSame(left.right, right.left);
		}

	 }

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