Difficulty: Medium
More:【目录】LeetCode Java实现
Description
https://leetcode.com/problems/binary-tree-level-order-traversal/
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] ]
Intuition
Using a queue.
Solution
public List<List<Integer>> levelOrder(TreeNode root) { List<List<Integer>> wrapList = new LinkedList<List<Integer>>(); LinkedList<TreeNode> queue = new LinkedList<>(); if(root!=null) queue.offer(root); while(!queue.isEmpty()){ int num = queue.size(); List<Integer> subList = new LinkedList<Integer>(); for(int i=0; i<num; i++){ TreeNode node = queue.poll(); subList.add(node.val); if(node.left!=null) queue.offer(node.left); if(node.right!=null) queue.offer(node.right); } wrapList.add(subList); } return wrapList; }
Complexity
Time complexity : O(n)
Space complexity : O(n)
More:【目录】LeetCode Java实现