feat: add 107

Author: Blankj

README.md

@@ -27,6 +27,9 @@
 |83|[Remove Duplicates from Sorted List][083]|Linked List|
 |88|[Merge Sorted Array][088]|Array, Two Pointers|
 |100|[Same Tree][100]|Tree, Depth-first Search|
+|101|[Symmetric Tree][101]|Tree, Depth-first Search, Breadth-first Search|
+|104|[Maximum Depth of Binary Tree][104]|Tree, Depth-first Search|
+|107|[Binary Tree Level Order Traversal II][107]|Tree, Breadth-first Search|
 
 
 ## Medium
@@ -71,6 +74,9 @@
 [083]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/083/README.md
 [088]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/088/README.md
 [100]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/100/README.md
+[101]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/101/README.md
+[104]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/104/README.md
+[107]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/107/README.md
 
 [008]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/008/README.md
 [019]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/019/README.md

note/107/README.md

@@ -0,0 +1,72 @@
+# [Binary Tree Level Order Traversal II][title]
+
+## Description
+
+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]
+]
+```
+
+**Tags:** Tree, Breadth-first Search
+
+
+## 思路
+
+题意是从下往上按层遍历二叉树，每一层是从左到右，由于是从下往上，那么我们就先遍历，当链表尺寸小于深度的时候，我们在链表首部插入新的节点，然后在最后遍历完插入节点即可，好好模拟想一想，画一画，应该能想通。
+
+``` java
+/**
+ * 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<>();
+        helper(list, root, 0);
+        return list;
+    }
+
+    private void helper(List<List<Integer>> list, TreeNode root, int level) {
+        if (root == null) return;
+        if (level >= list.size()) {
+            list.add(0, new LinkedList<>());
+        }
+        helper(list, root.left, level + 1);
+        helper(list, root.right, level + 1);
+        list.get(list.size() - level - 1).add(root.val);
+    }
+}
+```
+
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我GitHub上的LeetCode题解：[awesome-java-leetcode][ajl]
+
+
+
+[title]: https://leetcode.com/problems/binary-tree-level-order-traversal-ii
+[ajl]: https://github.com/Blankj/awesome-java-leetcode