Simplify the code in array.swift (#960)

* 简单点

* Apply suggestions from code review

Co-authored-by: nuomi1 <nuomi1@qq.com>

---------

Co-authored-by: Yudong Jin <krahets@163.com>
Co-authored-by: nuomi1 <nuomi1@qq.com>

codes/swift/chapter_array_and_linkedlist/array.swift

@@ -28,7 +28,7 @@ func extend(nums: [Int], enlarge: Int) -> [Int] {
 /* 在数组的索引 index 处插入元素 num */
 func insert(nums: inout [Int], num: Int, index: Int) {
     // 把索引 index 以及之后的所有元素向后移动一位
-    for i in sequence(first: nums.count - 1, next: { $0 > index + 1 ? $0 - 1 : nil }) {
+    for i in nums.indices.dropFirst(index).reversed() {
         nums[i] = nums[i - 1]
     }
     // 将 num 赋给 index 处元素
@@ -37,9 +37,8 @@ func insert(nums: inout [Int], num: Int, index: Int) {
 
 /* 删除索引 index 处元素 */
 func remove(nums: inout [Int], index: Int) {
-    let count = nums.count
     // 把索引 index 之后的所有元素向前移动一位
-    for i in sequence(first: index, next: { $0 < count - 1 - 1 ? $0 + 1 : nil }) {
+    for i in nums.indices.dropFirst(index).dropLast() {
         nums[i] = nums[i + 1]
     }
 }