103. Binary Tree Zigzag Level Order Traversal

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]
]

题目含义：给定一棵二叉树，从上到下，进行Z字行分层遍历的，即：本层的话从左打到右，那么下层就是从右到左，最后输出结果。

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
       List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        
        boolean shunshizhen = false;
        while (!q.isEmpty())
        {
            int size = q.size();
            List<Integer> values = new ArrayList<>();
            for (int i=0;i<size;i++)
            {
                TreeNode node  = q.poll();
                values.add(node.val);
                if (shunshizhen)
                {
                    if (node.left !=null) q.offer(node.left);
                    if (node.right !=null) q.offer(node.right);      
                }else
                {
                    if (node.right !=null) q.offer(node.right);
                    if (node.left !=null) q.offer(node.left);
                }
            }
            shunshizhen = shunshizhen?false:true;
            result.add(values);
        }
        return result;        
    }