Binary Tree Zigzag Level Order Traversal，z字形遍历二叉树，得到每层访问的节点值。

问题描述：

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,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

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

算法分析：和前面问题类似。

public class BinaryTreeZigzagLevelOrderTraversal 
{
	 public List<List<Integer>> zigzagLevelOrder(TreeNode root) 
	 {
		 List<Integer> list = new ArrayList<>();
		 List<List<Integer>> res = new ArrayList<>();
		 
		 if(root == null) 
		 {
			 return res;
		 }
		 
		 Stack<TreeNode> s1 = new Stack<>();
		 Stack<TreeNode> s2 = new Stack<>();
		 
		 s1.push(root);
		 while(!s1.isEmpty() || !s2.isEmpty())
		 {
			 while(!s1.isEmpty())
			 {
				 TreeNode temp = s1.pop();
				 if(temp.left != null)
				 {
					 s2.push(temp.left);
				 }
				 if(temp.right != null)
				 {
					 s2.push(temp.right);
				 }
				 list.add(temp.val);
			 }
			 if(list.size() != 0)
				 res.add(list);
			 list = new ArrayList<>();
			 while(!s2.isEmpty())
			 {
				 TreeNode temp = s2.pop();
				 if(temp.right != null)
				 {
					 s1.push(temp.right);
				 }
				 if(temp.left != null)
				 {
					 s1.push(temp.left);
				 }
				 list.add(temp.val);
			 }
			 if(list.size() != 0)
				 res.add(list);
			 list = new ArrayList<>();
		 }
		 return res;
	 }
}