107. Binary Tree Level Order Traversal II
Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).
Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: [[15,7],[9,20],[3]]
Example 2:
Input: root = [1]
Output: [[1]]
Example 3:
Input: root = []
Output: []
Constraints:
- The number of nodes in the tree is in the range [0, 2000].
- -1000 <= Node.val <= 1000
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> list = new LinkedList<>();
Queue<TreeNode> queue = new LinkedList<>();
if(root==null){
return list;
}
queue.add(root);
while(!queue.isEmpty()){
int size = queue.size();
List<Integer> temp = new ArrayList();
for(int i=0;i<size;i++){
TreeNode node = queue.poll();
temp.add(node.val);
if(node.left != null){
queue.add(node.left);
}
if(node.right != null){
queue.add(node.right);
}
}
list.add(0, temp);
}
return list;
}
}