题解 | #按之字形顺序打印二叉树#

一个队列实现，分奇偶层，偶数层反转一下。

import java.util.ArrayList;
/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode p1;
        ArrayList<ArrayList<Integer> > re = new ArrayList<>();
        ArrayList<Integer> list = new ArrayList<>();
        queue.offer(pRoot);
        int size;
        int k=0;
        if(pRoot == null){
            return re;
        }
        while(!queue.isEmpty()){
            size = queue.size();
            k++;
            while(size>0){
                size--;
                p1 = queue.poll();
                list.add(p1.val);
                if(p1.left != null ){
                    queue.offer(p1.left);
                }
                if(p1.right != null ){
                    queue.offer(p1.right);
                }
            }
            if(list.size() != 0){
                if(k % 2 ==0){
                     Collections.reverse(list);
                     re.add(new ArrayList(list));
                }else{
                    re.add(new ArrayList(list));
                }
            }
            list.clear();
            
        }
        
        return re;
        
        
    }

}