题解 | #按之字形顺序打印二叉树#

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.offer(pRoot);
        int level = 1;
        while(!queue.isEmpty()){
            ArrayList<Integer> list = new ArrayList<Integer>();
            int levelLen = queue.size();
            for (int i = 0; i < levelLen; i++) {
                TreeNode node = queue.poll();
                if((level % 2) != 0){ // 奇数层 正序
                    list.add(node.val) ;
                }else{
                    // 偶数层 逆序
                    list.add(0,node.val);
                }

                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            level++;
            res.add(new ArrayList<>(list));
        }
        return res;
    }

}