BFS场景样例测试--层次遍历--用Java实现

我们今天研究下层次遍历用BFS来实现

 

首先确定数据结构

/**
 * 二叉树数据结构节点
 */
public class TreeNode {

    int value;
    TreeNode left;
    TreeNode right;

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

　　然后走下场景的代码

 

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;

public class BFSDemo {

    /***
     * 层次遍历
     * @param root
     * @return
     */
    public List<List<Integer>> levelOrder(TreeNode root){
        Queue<TreeNode>  q = new ArrayDeque<>();

        // 将根节点加入队列
        if(null != root){
            q.offer(root);
        }

        List<List<Integer>> res = new ArrayList<>();

        while (!q.isEmpty()){
            int sz = q.size();

            // TODO  这里有优化空间 暂时不修改
            List<Integer> num = new ArrayList<>();

            for (int i = 0; i < sz; i++) {
                TreeNode cur = q.poll();

                // 将左右节点加入队列 以备下一轮迭代
                if(cur.left!=null){
                    q.offer(cur.left);
                }

                if(cur.right!=null){
                    q.offer(cur.right);
                }

                num.add(cur.value);

            }
            res.add(num);

        }
        return res;
    }

    public static void main(String[] args){

        // 构建测试数据

        TreeNode root = new TreeNode(9);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node7 = new TreeNode(7);
        TreeNode node5 = new TreeNode(5);
        TreeNode node4 = new TreeNode(4);

        root.left = node1;
        root.right = node2;
        node2.left = node7;
        node2.right = node5;
        node7.left = node4;

        BFSDemo bfsDemo = new BFSDemo();

        List<List<Integer>> list = bfsDemo.levelOrder(root);

        if(null==list){
            return;
        }
        //System.out.println(list.toString());
        for (int i = 0; i < list.size(); i++) {
            System.out.print("[");
            if(null !=list.get(i) && list.get(i).size()!=0) {
                for (int j = 0; j < list.get(i).size(); j++) {
                    System.out.print( " "+list.get(i).get(j)+" ");
                }
            }
            System.out.println("]");
        }
    }
}

　　执行main run

 

结果