层遍历问题
问题：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).
Input:
- 树的跟 :: TreeNode
Output:
- 整棵树的按层遍历：List<List<Integer>>
Intuition:
这个题可以做BFS的例题了，就酱...
Code:
TC:O()
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null){
return res;
}
Deque<TreeNode> que = new ArrayDeque<>();
que.offer(root);
while (!que.isEmpty()){
List<Integer> list = new ArrayList<>();
int size = que.size();
for(int i = 0; i < size; i++){ //nodes in one level
ListNode cur = que.poll();
list.add(cur.val);
if (cur.left != null){
que.offer(cur.left);
}
if (cur.right != null){
que.offer(cur.right);
}
}
res.add(list);
}
return res;
}
