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 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pRoot TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        // write code here
         ArrayList<ArrayList<Integer>> aal=new ArrayList<>();
         dfs(pRoot,aal,0);//直接层序遍历
         for(int i=1;i<aal.size();i=i+2){//偶数层换一下顺序就ok
            Collections.reverse(aal.get(i));
         }
         return aal;


    }
    public void dfs (TreeNode pRoot,ArrayList<ArrayList<Integer>> aal,int level){
        if(pRoot==null){
            return;
        }
        if(aal.size()==level){
            aal.add(new ArrayList<Integer>());
        }
         aal.get(level).add(pRoot.val);
         dfs(pRoot.left,aal,level+1);
         dfs(pRoot.right,aal,level+1);
        return;
    }
    
}