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。

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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;      } }