题解 | #按之字形顺序打印二叉树#

import java.util.ArrayList;
import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        if(pRoot == null){
            return res;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        boolean isLeft = true;
        while(!queue.isEmpty()){
            int size = queue.size();
            ArrayList<Integer> list = new ArrayList<>();
            for(int i = 0; i < size; ++i){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null){queue.add(node.left);}
                if(node.right != null){queue.add(node.right);}
            }
            if(isLeft){
                res.add(list);
            }else{
                Collections.reverse(list);
                res.add(list);
            }
            isLeft = !isLeft;
        }

        return res;
    }

}