二叉树的广度优先搜索BFS（两种实现方法）

import java.util.HashMap;
import java.util.LinkedList;
import java.util.Queue;

public class test15 {
    public static class Node{
        public int value;
        public Node left;
        public Node right;

        public Node(int data){
            this.value = data;
        }
    }


//按层次遍历二叉树并打印每个节点的值，使用了队列来实现广度优先搜索（BFS）
    public static void level(Node head){
        if(head == null){
            return;
        }
        Queue<Node> queue = new LinkedList<>();
        queue.add(head);
        while (!queue.isEmpty()){
            Node cur = queue.poll();
            System.out.println(cur.value);
            if(cur.left != null){
                queue.add(cur.left);
            }
            if(cur.right != null){
                queue.add(cur.right);
            }
        }
    }


    //求树的最大宽度
    //方法1使用HashMap
    public static int maxWidthUseMap(Node head){
        if(head == null){
            return 0;
        }
        Queue<Node> queue= new LinkedList<>();
        queue.add(head);
        //key在哪一层为value的值
        HashMap<Node , Integer> levelMap= new HashMap<>();
        levelMap.put(head ,1);
        int curLevel = 1;//当前统计哪一层的宽度
        int curLevelNodes = 0;//当前层的宽度为多少
        int max = 0;
        while (!queue.isEmpty()){
            Node cur = queue.poll();
            int curNodeLevel = levelMap.get(cur);
            if(cur.left != null){
                levelMap.put(cur.left  ,curNodeLevel +1);
                queue.add(cur.left);
            }
            if(cur.right != null){
                levelMap.put(cur.left , curNodeLevel + 1);
                queue.add(cur.right);
            }

            if(curNodeLevel == curLevel){
                curLevelNodes++;
            }else{
                max = Math.max(max , curLevelNodes);
                curLevel++;
                curLevelNodes =1;
            }
        }
        max = Math.max(max , curLevelNodes);
        return  max;
    }
    //方法2不用HashMap
    public static int  maxWidthNoMap(Node head){
        if(head == null){
            return 0;
        }
        Queue<Node> queue = new LinkedList<>();
        queue.add(head);
        Node curEnd = head;//当前层最后一个节点是谁
        Node nextEnd = null;//如果有下一层，那么下一层最后节点是谁
        int max =0;
        int curLevelNodes = 0;//当前层的节点数
        while (!queue.isEmpty()){
            Node cur = queue.poll();
            if(cur.left != null){
                queue.add(cur.left);
                nextEnd = cur.left;
            }
            if(cur.right != null){
                queue.add(cur.right);
                nextEnd = cur.right;
            }
            curLevelNodes++;
            if(cur == curEnd){
                max = Math.max(max , curLevelNodes);
                curLevelNodes = 0;
                curEnd = nextEnd;
            }
        }
        return max;
    }
}