Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
题意：按层级倒序遍历二叉树。
思路：这道题是102题的followup，要求从正序遍历变为倒序，所以只需要在102题的解法上加上一行reverse代码即可。
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> q = new LinkedList<>();
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
List<Integer> list = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode cur = q.poll();
list.add(cur.val);
if (cur.left != null) {
q.offer(cur.left);
}
if (cur.right != null) {
q.offer(cur.right);
}
}
res.add(list);
}
//翻转，逆序输出
Collections.reverse(res);
return res;
}