Binary Tree Level Order Traversal II
Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values, from left to right, level by level from leaf to root.
Example 1
3
/ \
9 20
/ \
15 7Input
root = [3,9,20,null,null,15,7]Output
[[15,7],[9,20],[3]]The traversal lists the leaf level
[15, 7] first, then [9, 20], and finally the root level [3].Example 2
1
Input
root = [1]Output
[[1]]The tree has only one node, so the bottom-up level order traversal is
[[1]].Constraints
- The number of nodes in the tree is in the range
[0, 2000]. -1000 <= Node.val <= 1000