feat: azl397985856#92 增加注释 #236增加扩展 backlog增加题目

backlog/538.convert-bst-to-greater-tree.js

@@ -0,0 +1,61 @@
+/*
+ * @lc app=leetcode id=538 lang=javascript
+ *
+ * [538] Convert BST to Greater Tree
+ *
+ * https://leetcode.com/problems/convert-bst-to-greater-tree/description/
+ *
+ * algorithms
+ * Easy (50.04%)
+ * Total Accepted:    75.4K
+ * Total Submissions: 149K
+ * Testcase Example:  '[5,2,13]'
+ *
+ * Given a Binary Search Tree (BST), convert it to a Greater Tree such that
+ * every key of the original BST is changed to the original key plus sum of all
+ * keys greater than the original key in BST.
+ *
+ *
+ * Example:
+ *
+ * Input: The root of a Binary Search Tree like this:
+ * ⁠             5
+ * ⁠           /   \
+ * ⁠          2     13
+ *
+ * Output: The root of a Greater Tree like this:
+ * ⁠            18
+ * ⁠           /   \
+ * ⁠         20     13
+ *
+ *
+ */
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {TreeNode}
+ */
+var convertBST = function(root) {
+  let res = 0;
+  function r(root) {
+    if (root === null) return null;
+
+    r(root.right);
+
+    root.val += res;
+
+    res = +root.val;
+
+    r(root.left);
+
+    return root;
+  }
+  r(root);
+  return root;
+};

problems/236.lowest-common-ancestor-of-a-binary-tree.md

@@ -147,3 +147,6 @@ var lowestCommonAncestor = function(root, p, q) {
 };
 ```
 
+## 扩展
+如果递归的结束条件改为`if (!root || root.left === p || root.right === q) return root;` 代表的是嗯呢么意思，对结果有什么样的影响？
+

problems/92.reverse-linked-list-ii.md

@@ -89,14 +89,16 @@ Output: 1->4->3->2->5->NULL
  * @return {ListNode}
  */
 var reverseBetween = function(head, m, n) {
+    // 虚拟节点，简化操作
     const dummyHead = {
         next: head
     }
 
-    let current = dummyHead.next;
-    let pre = current;
-    let index = 0;
+    let current = dummyHead.next; // 当前遍历的节点
+    let pre = current; // 因为要反转，因此我们需要记住前一个节点
+    let index = 0; // 链表索引，用来判断是否是特殊位置（头尾位置）
 
+    // 上面提到的四个特殊节点
     let midStartNode = null;
     let preMidStartNode = null;
     let midEndNode = null;
@@ -106,11 +108,12 @@ var reverseBetween = function(head, m, n) {
         const next = current.next;
         index++;
 
-        // reverse linked list
+        // 对 (m - n) 范围内的节点进行反转
         if (index > m && index <= n) {           
             current.next = pre;
         }
 
+        // 下面四个if都是边界, 用于更新四个特殊节点的值
         if (index === m - 1) {
             preMidStartNode = current;
         }
@@ -120,7 +123,6 @@ var reverseBetween = function(head, m, n) {
 
         if (index === n + 1) {
             postMidEndNode = current;
-            postMidEndNode.next
         }
 
         if (index === n) {