LeetCode刷题实战107：二叉树的层次遍历 II
算法的重要性，我就不多说了吧，想去大厂，就必须要经过基础知识和业务逻辑面试+算法面试。所以，为了提高大家的算法能力，这个公众号后续每天带大家做一道算法题，题目就从LeetCode上面选 ！
今天和大家聊的问题叫做 二叉树的层次遍历 II，我们先来看题面：
https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/
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).
题意
解题
class Solution {
public List<List> levelOrderBottom(TreeNode root) {
List<List> levelOrder = new LinkedList<List >();
if (root == null) {
return levelOrder;
}
Queuequeue = new LinkedList ();
queue.offer(root);
while (!queue.isEmpty()) {
Listlevel = new ArrayList ();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
level.add(node.val);
TreeNode left = node.left, right = node.right;
if (left != null) {
queue.offer(left);
}
if (right != null) {
queue.offer(right);
}
}
levelOrder.add(0, level);
}
return levelOrder;
}
}
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