102. 二叉树的层序遍历

题目链接

未优化代码

    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        Queue<TreeNode> q2 = new LinkedList<>();

        q.add(root);

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

        if (root == null) {
            return list;
        }

        List<Integer> temp;
        TreeNode treeNode;

        boolean flag;
        while (!q.isEmpty() || !q2.isEmpty()) {
            temp = new ArrayList<>();

            flag = false;

            // 下面这两个循环只能有一个起作用
            while (!q.isEmpty()) {
                flag = true;
                treeNode = q.poll();
                temp.add(treeNode.val);

                if (treeNode.left != null) {
                    q2.add(treeNode.left);
                }

                if (treeNode.right != null) {
                    q2.add(treeNode.right);
                }
            }
            while (!q2.isEmpty() && flag == false) {
                treeNode = q2.poll();
                temp.add(treeNode.val);

                if (treeNode.left != null) {
                    q.add(treeNode.left);
                }

                if (treeNode.right != null) {
                    q.add(treeNode.right);
                }
            }

            list.add(temp);
        }

        return list;
    }

优化代码来自

        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        if (root == null) {
            return ret;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<Integer>();
            int currentLevelSize = queue.size();
            for (int i = 1; i <= currentLevelSize; ++i) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            ret.add(level);
        }

        return ret;
    }