二叉树的层次遍历

题目原型：

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

略

	public ArrayList
  
   > levelOrder(TreeNode root)
	{
		ArrayList
   
    > list = new ArrayList
    
     >(); ArrayList
     
       nodeSet = new ArrayList
      
       (); ArrayList
       
         tmp ; ArrayList
        
          numSet ; if(root!=null) { nodeSet.add(root); while(nodeSet.size()>0) { tmp = new ArrayList
         
          (); numSet = new ArrayList
          
           (); //添加到list中 for(TreeNode tn : nodeSet) numSet.add(tn.val); list.add(numSet); //求下一层的节点 for(TreeNode it : nodeSet) { if(it.left!=null) tmp.add(it.left); if(it.right!=null) tmp.add(it.right); } nodeSet = tmp; } } return list; }