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;
}
}