题目链接
https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/
题目描述
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]
]
题解
层次遍历，用队列存起来。
代码
/**
* 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<>();
if (root == null) { return list; }
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
List<Integer> subList = new LinkedList<>();
for (int i = 0; i <size; i++) {
if (queue.peek().left != null) { queue.offer(queue.peek().left); }
if (queue.peek().right != null) { queue.offer(queue.peek().right); }
subList.add(queue.poll().val);
}
list.add(0, subList);
}
return list;
}
}