层序遍历二叉树的两种方法

第一种也是最常用的一种，使用queue。还有一种不使用queue的方法。不使用queue的思路，其实就是每次都只存储一层的节点，然后遍历这一层的节点，是真正的按层遍历的思想。每次遍历的都是当前层，记录的都是当前层的下一层。

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public class test {     public static void main(String[] args)     {         TreeNode root = new TreeNode(1);         TreeNode r1 = new TreeNode(2);         TreeNode r2 = new TreeNode(3);         root.left = r1;         root.right = r2;         test.levelTraversal2(root);     }     //不使用队列     public static void levelTraversal(TreeNode root)     {         ArrayList<TreeNode> list = new ArrayList<>();         list.add(root);         while(!list.isEmpty())         {             ArrayList<TreeNode> temp = new ArrayList<>();             for (TreeNode node : list)             {                 System.out.print(node.val);                 if(node.left != null)                 {                     temp.add(node.left);                 }                 if(node.right != null)                 {                     temp.add(node.right);                 }             }             list = temp;         }     }           //使用队列     public static void levelTraversal2(TreeNode root)     {         ArrayDeque<TreeNode> queue = new ArrayDeque<>();         queue.add(root);         while(!queue.isEmpty())         {             TreeNode temp = queue.poll();             System.out.print(temp.val);             if(temp.left != null)             {                 queue.offer(temp.left);             }             if(temp.right != null)             {                 queue.offer(temp.right);             }         }     } }