Leetcode：剑指 Offer 32 - III. 从上到下打印二叉树 III

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res=new ArrayList<List<Integer>>();
        if(root==null){
            return res;
        }
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            //使用LinkedList链表可以选择进行头插法还是尾插法，方便旋转
            LinkedList<Integer> level=new LinkedList<Integer>();
            int queuesize=queue.size();
            for(int i=0;i<queuesize;i++){
                TreeNode node=queue.poll();
                int resSize=res.size();
                //树的根节点是第0层，所以这里是偶数层，正常插入
                if(resSize%2==0){
                    level.addLast(node.val);
                }
                //奇数层反着插入
                else{
                    level.addFirst(node.val);
                }
                if(node.left!=null){
                    queue.offer(node.left);
                }
                if(node.right!=null){
                    queue.offer(node.right);
                }
            }
            res.add(level);
        }
        return res;
    }
}