按之字形顺序打印二叉树

常规的层序遍历解法：

import java.util.*;

public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode root) {
        ArrayList<ArrayList<Integer>> ret = new ArrayList<>();
        if (root == null) 
            return ret;
        Queue<TreeNode> queue = new LinkedList<>();
        boolean rev = false;
        queue.offer(root);
        while(!queue.isEmpty()) {
            int len = queue.size();
            ArrayList<Integer> list = new ArrayList<>(len);
            while((len--) > 0) {
                TreeNode cur = queue.poll();
                if (rev) {
                    list.add(0, cur.val);
                } else {
                    list.add(cur.val);
                }
                if (cur.left != null) {
                    queue.offer(cur.left);
                }
                if (cur.right != null) {
                    queue.offer(cur.right);
                }
            }
            ret.add(list);
            rev = !rev; 
        }
        return ret;
    }
}