二叉树层序遍历（宽度优先遍历bfs--队列）

https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/solution/mian-shi-ti-32-ii-cong-shang-dao-xia-da-yin-er-c-5/

剑指 Offer 32 - II. 从上到下打印二叉树 II

public List<List<Integer>> levelOrder(TreeNode root) {
        //队列，遍历上一层的时候，顺便吧下一层元素加入队列
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> tmp = new LinkedList<>();
            int lastLen = queue.size();
            for (int i = lastLen; i > 0; i--) {
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if (node.left != null) {//把下一层元素放入队列
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            ans.add(tmp);
        }
        return ans;
    }

https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/

剑指 Offer 32 - I. 从上到下打印二叉树

public  int[] levelOrder(TreeNode root) {
        //队列
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> list = new LinkedList<>();
        if (root == null) {
            return new int[]{};
        }
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            list.add(node.val);
            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }
        }
        int[] ans = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            ans[i] = list.get(i);
        }
        return ans;

    }

https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/ 

剑指 Offer 32 - III. 从上到下打印二叉树 III

    public List<List<Integer>> levelOrder(TreeNode root) {
        //队列，遍历上一层的时候，顺便吧下一层元素加入队列
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }
        queue.add(root);
        int count = 1;
        while (!queue.isEmpty()) {
            List<Integer> tmp = new LinkedList<>();
            int lastLen = queue.size();
            for (int i = lastLen; i > 0; i--) {
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if (node.left != null) {//把下一层元素放入队列
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            if (count % 2 == 0) {
                Collections.reverse(tmp);
            }
            ans.add(tmp);
            count = count + 1;
        }
        return ans;
    }