diff --git a/problems/0027.移除元素.md b/problems/0027.移除元素.md
index e1de7243..a563a13f 100644
--- a/problems/0027.移除元素.md
+++ b/problems/0027.移除元素.md
@@ -199,21 +199,15 @@ Python：
 ```python3
 class Solution:
     def removeElement(self, nums: List[int], val: int) -> int:
-        if nums is None or len(nums)==0: 
-            return 0 
-        l=0
-        r=len(nums)-1
-        while l<r: 
-            while(l<r and nums[l]!=val):
-                l+=1
-            while(l<r and nums[r]==val):
-                r-=1
-            nums[l], nums[r]=nums[r], nums[l]
-        print(nums)
-        if nums[l]==val: 
-            return l 
-        else: 
-            return l+1
+        # 快指针遍历元素
+        fast = 0
+        # 慢指针记录位置
+        slow = 0
+        for fast in range(len(nums)):
+            if nums[fast] != val:
+                nums[slow] = nums[fast]
+                slow += 1
+        return slow
 ```
 
 
diff --git a/problems/0209.长度最小的子数组.md b/problems/0209.长度最小的子数组.md
index c912c259..220630f2 100644
--- a/problems/0209.长度最小的子数组.md
+++ b/problems/0209.长度最小的子数组.md
@@ -169,39 +169,18 @@ Python：
 ```python
 class Solution:
     def minSubArrayLen(self, s: int, nums: List[int]) -> int:
-        # 定义一个无限大的数
-        res = float("inf")
-        Sum = 0
-        index = 0
-        for i in range(len(nums)):
-            Sum += nums[i]
+        res = float("inf")   # 定义一个无限大的数
+        Sum = 0     # 滑动窗口数值之和
+        i = 0      # 滑动窗口起始位置
+        for j in range(len(nums)):
+            Sum += nums[j]
             while Sum >= s:
-                res = min(res, i-index+1)
-                Sum -= nums[index]
-                index += 1
-        return 0 if res==float("inf") else res
-```
-```python
-# 滑动窗口
-class Solution:
-    def minSubArrayLen(self, target: int, nums: List[int]) -> int:
-        if nums is None or len(nums) == 0: 
-            return 0 
-        lenf = len(nums) + 1
-        total = 0
-        i = j = 0
-        while (j < len(nums)):
-            total = total + nums[j]
-            j += 1
-            while (total >= target):
-                lenf = min(lenf, j - i)
-                total = total - nums[i]
+                res = min(res, j-i+1)
+                Sum -= nums[i]
                 i += 1
-        if lenf == len(nums) + 1:
-            return 0
-        else: 
-            return lenf
+        return 0 if res == float("inf") else res
 ```
+
 Go：
 ```go
 func minSubArrayLen(target int, nums []int) int {
@@ -232,22 +211,23 @@ func minSubArrayLen(target int, nums []int) int {
 JavaScript:
 
 ```js
-
 var minSubArrayLen = function(target, nums) {
-    // 长度计算一次
-    const len = nums.length;
-    let l = r = sum = 0,
-        res = len + 1; // 子数组最大不会超过自身
-    while(r < len) {
-        sum += nums[r++];
-        // 窗口滑动
-        while(sum >= target) {
-            // r始终为开区间 [l, r)
-            res = res < r - l ? res : r - l;
-            sum-=nums[l++];
+    let start, end
+    start = end = 0
+    let sum = 0
+    let len = nums.length
+    let ans = Infinity
+    
+    while(end < len){
+        sum += nums[end];
+        while (sum >= target) {
+            ans = Math.min(ans, end - start + 1);
+            sum -= nums[start];
+            start++;
         }
+        end++;
     }
-    return res > len ? 0 : res;
+    return ans === Infinity ? 0 : ans
 };
 ```
 
diff --git a/problems/0707.设计链表.md b/problems/0707.设计链表.md
index 1264983b..9461e263 100644
--- a/problems/0707.设计链表.md
+++ b/problems/0707.设计链表.md
@@ -108,9 +108,12 @@ public:
     // 如果index大于链表的长度，则返回空
     // 如果index小于0，则置为0，作为链表的新头节点。
     void addAtIndex(int index, int val) {
-        if (index > _size || index < 0) {
+        if (index > _size) {
             return;
         }
+	if (index < 0) {
+            index = 0;
+        }
         LinkedNode* newNode = new LinkedNode(val);
         LinkedNode* cur = _dummyHead;
         while(index--) {