Binary Tree Zigzag Level Order Traversal （二叉树锯齿形层次遍历）

题目原型：

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

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

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

基本思路：

这个题目比较好想。

	public ArrayList> zigzagLevelOrder(TreeNode root)
	{
		ArrayList> list = new ArrayList>();
		ArrayList nodeSet = new ArrayList();
		ArrayList tmp;
		ArrayList numSet;
		int count = 1;
		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,注意是从后往前遍历
				for (int i = nodeSet.size()-1;i>=0;i--)
				{
					if (count % 2 == 0)
					{
						if (nodeSet.get(i).left != null)
							tmp.add(nodeSet.get(i).left);
						if (nodeSet.get(i).right != null)
							tmp.add(nodeSet.get(i).right);
					} else
						//如果位于奇数层，则从右孩子开始添加
					{
						if (nodeSet.get(i).right != null)
							tmp.add(nodeSet.get(i).right);
						if (nodeSet.get(i).left != null)
							tmp.add(nodeSet.get(i).left);
					}
				}
				nodeSet = tmp;
				count++;
			}
		}
		return list;
	}