二叉树的层序遍历

二叉树的层序遍历

利用队列的先进先出的特性，实现二叉树的层序遍历

//# 二叉树的层序正序遍历
public  static List<List<Integer>> 
    levelOrder(Node root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        //创建优先级队列
        Queue<Node> queue = new ArrayDeque<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                Node temp = queue.poll();
                list.add(temp.val);
                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }
            result.add(list);
        }
        return result;
    }

//# 二叉树的层序反序遍历
public static List<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        //创建优先级队列
        Queue<Node> queue = new ArrayDeque<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                Node temp = queue.poll();
                list.add(temp.val);
                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }
            result.add(list);
        }
        Collections.reverse(result);
        return result;
    }

//二叉树的右视图
public static List<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        //创建优先级队列
        Queue<Node> queue = new ArrayDeque<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                Node temp = queue.poll();
                //二叉树右视图的体现
                if (i == size - 1) {
                    list.add(temp.val);
                }

                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }
            result.add(list);
        }
        return result;
    }

//二叉树的层平均值
public  static  List<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        //创建优先级队列
        Queue<Node> queue = new ArrayDeque<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            int sum = 0;//统计每一层的值
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                Node temp = queue.poll();
                sum += temp.val;
                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }
            //将平均值添加到集合中
            list.add(sum/size);
            result.add(list);
        }
        return result;
    }

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