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>> list = new ArrayList<>();
        level(pRoot,0,list);
        for(int i = 1; i < list.size(); i+=2){
            Collections.reverse(list.get(i));
        }
        return list;
    }

    private void level(TreeNode pRoot, int i, ArrayList<ArrayList<Integer>> list) {
        // TODO
        if(pRoot == null){
            return;
        }
        if(list.size()<=i){
            list.add(new ArrayList<Integer>());
        }
        list.get(i).add(pRoot.val);
        level(pRoot.left,i+1,list);
        level(pRoot.right,i+1,list);
    }
}