102. Binary Tree Level Order Traversal(Java)

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Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example: Given binary tree [3,9,20,null,null,15,7],

3 / \ 9 20 / \ 15 7

return its level order traversal as:

[ [3], [9,20], [15,7] ] /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public List<List<Integer>> levelOrder(TreeNode root) { Queue<TreeNode> queue = new LinkedList<>(); List<List<Integer>> wrapList = new LinkedList<>(); TreeNode cur = root; if (cur == null) return wrapList; queue.offer(cur); while(!queue.isEmpty()) { int levelNum = queue.size(); List<Integer> subList = new LinkedList<>(); for (int i = 0; i < levelNum; i ++) { if (queue.peek().left != null) queue.offer(queue.peek().left); if (queue.peek().right != null) queue.offer(queue.peek().right); subList.add(queue.poll().val); } wrapList.add(subList); } return wrapList; } }