题解 | #按之字形顺序打印二叉树#

import java.util.*;

public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer> > result = new ArrayList<>();
        if(pRoot==null){
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        boolean isLeft = true;
        while(!queue.isEmpty()){
            int count = queue.size();
            ArrayList<Integer> list = new ArrayList<>();
            for(int i = 0; i<count; i++){
                TreeNode poll = queue.poll();
                if(poll.left!=null){
                    queue.add(poll.left);
                }
                if(poll.right!=null){
                    queue.add(poll.right);
                }
                list.add(poll.val);
            }
            if(isLeft){
                result.add(list);
            }else{
                ArrayList<Integer> listR = new ArrayList<>(list.size());
                for (int i = list.size() - 1; i >= 0; i--) {
                    listR.add(list.get(i));
                }
                result.add(listR);
            }
            isLeft = !isLeft;
        }
        return result;
    }

}