102. Binary Tree Level Order Traversal

Solution 1:

class Solution {

    public List<List<Integer>> levelOrder(TreeNode root) {

        Queue<TreeNode> queue = new LinkedList<>();

        List<List<Integer>> wrapList = new LinkedList<>();

        if(root == null) 

            return wrapList;

        queue.offer(root);

        while (!queue.isEmpty()) {

            int levelNum = queue.size();

            List<Integer> subList = new LinkedList<>();

            for(int i = 0; i < levelNum; i++) {

            　 TreeNode n = q.poll();　　

　　　　    if(n.left != null) q.add(n.left);

                if(n.right != null) q.add(n.right);

                subList.add(n.val);

            }

            wrapList.add(subList);

        }

        return wrapList;

    }

}

 

//如果要求把null节点也列出来：

public static List<List<Integer>> levelOrder(TreeNode root) {

    Queue<TreeNode> queue = new LinkedList<>();

    List<List<Integer>> wrapList = new LinkedList<>();

    if(root == null)

        return wrapList;

    queue.offer(root);

    while (!queue.isEmpty()) {

        int levelNum = queue.size();

        List<Integer> subList = new LinkedList<>();

        for(int i = 0; i < levelNum; i++) {

            TreeNode tn = queue.poll();

            if(tn != null)

                queue.offer(tn.left);

            if(tn != null)

                queue.offer(tn.right);

            if (tn == null)

                subList.add(null);

            else

                subList.add(tn.val);

        }

        wrapList.add(subList);

    }

    return wrapList;

}

 

 

Solution 2:

class Solution {

   public List<List<Integer>> levelOrder(TreeNode root) {

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

        levelHelper(res, root, 0);

        return res;

    }

    

    public void levelHelper(List<List<Integer>> res, TreeNode root, int height) {

        if (root == null) return;

        if (height >= res.size()) {

            res.add(new LinkedList<>());

        }

        res.get(height).add(root.val);

        levelHelper(res, root.left, height+1);

        levelHelper(res, root.right, height+1);

    }

}