Update 0001.两数之和.md

Author: buptz2b

problems/0001.两数之和.md

@@ -287,17 +287,16 @@ func twoSum(nums []int, target int) []int {
 ```
 
 ```go
-// 使用map方式解题，降低时间复杂度
 func twoSum(nums []int, target int) []int {
     m := make(map[int]int)
-    for index, val := range nums {
-        if preIndex, ok := m[target-val]; ok {
-            return []int{preIndex, index}
-        } else {
-            m[val] = index
+    for i, num := range nums {
+        complement := target - num
+        if index, found := m[complement]; found {
+            return []int{index, i}
         }
+        m[num] = i
     }
-    return []int{}
+    return nil  // 返回空数组 nil 代替空切片
 }
 ```