【牛客网-名企高频面试题】 NC15 求二叉树的层序遍历

NC15 求二叉树的层序遍历

import java.util.*;

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

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        if(root == null){
            return res;
        }
        count(root,0);
        return res;
    }
    public void count(TreeNode node,int level){
        if(level == res.size()){
            res.add(new ArrayList<Integer>());
        }
        ArrayList<Integer> list = res.get(level);
        list.add(node.val);
        
        if(node.left != null){
            count(node.left,level + 1);
        }
        if(node.right != null){
            count(node.right,level+1);
        }
    
    }
    
}