Question Definition
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]
]
Java Solution
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> result = new LinkedList<>();
if(root == null) return result;
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
queue.offer(null);
List<Integer> list = new LinkedList<>();
while(!queue.isEmpty()){
TreeNode cur = queue.poll();
if(cur == null){
if(queue.isEmpty()) break;
queue.offer(null);
result.add(list);
list = new LinkedList<>();
cur = queue.poll();
}
list.add(cur.val);
if(cur.left != null) queue.offer(cur.left);
if(cur.right != null) queue.offer(cur.right);
}
if(list.size() > 0) result.add(list);
return result;
}
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