树5：从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

Posted 2020-11-27 xuechengmeigui

分析：层次遍历的经典算法

import java.util.ArrayList;
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 {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer> >list=new ArrayList<ArrayList<Integer> >();
        if(pRoot==null) return list;
        int layer=1;
        Queue<TreeNode>q=new LinkedList<>();
        q.offer(pRoot);
        while(!q.isEmpty()){
            int count=q.size();
            ArrayList<Integer> temp=new ArrayList<>();
            while(count>0){
                TreeNode node=q.poll();
                temp.add(node.val);
                if(node.left!=null) q.offer(node.left);
                if(node.right!=null) q.offer(node.right);
                count--;
            }
            layer++;
            list.add(temp);
        }
        return list;
    }
    
}