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]
]
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
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> level = new ArrayList<>();
for(int i = 1; i <= size; i++) {
TreeNode current = q.poll();
level.add(current.val);
if(current.left != null)
q.offer(current.left);
if(current.right != null)
q.offer(current.right);
}
res.add(level);
}
Collections.reverse(res);
return res;
}
}
/**
* 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>> result = new ArrayList<List<Integer>>();
if(root == null){
return result;
}
Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
queue.offer(root);
while(!queue.isEmpty()){
int size = queue.size();
List<Integer> level = new ArrayList<Integer>();
for(int i = 0; i < size; i++){
TreeNode node = queue.poll();
level.add(node.val);
if(node.left != null)
queue.offer(node.left);
if(node.right != null)
queue.offer(node.right);
}
result.add(0,level);
}
return result;
}
}