feat: add Swift codes for counting_sort article (#438)

codes/swift/Package.swift

@@ -50,6 +50,7 @@ let package = Package(
         .executable(name: "insertion_sort", targets: ["insertion_sort"]),
         .executable(name: "quick_sort", targets: ["quick_sort"]),
         .executable(name: "merge_sort", targets: ["merge_sort"]),
+        .executable(name: "counting_sort", targets: ["counting_sort"]),
         .executable(name: "radix_sort", targets: ["radix_sort"]),
     ],
     targets: [
@@ -101,6 +102,7 @@ let package = Package(
         .executableTarget(name: "insertion_sort", path: "chapter_sorting", sources: ["insertion_sort.swift"]),
         .executableTarget(name: "quick_sort", path: "chapter_sorting", sources: ["quick_sort.swift"]),
         .executableTarget(name: "merge_sort", path: "chapter_sorting", sources: ["merge_sort.swift"]),
+        .executableTarget(name: "counting_sort", path: "chapter_sorting", sources: ["counting_sort.swift"]),
         .executableTarget(name: "radix_sort", path: "chapter_sorting", sources: ["radix_sort.swift"]),
     ]
 )

codes/swift/chapter_sorting/counting_sort.swift

@@ -0,0 +1,70 @@
+/**
+ * File: counting_sort.swift
+ * Created Time: 2023-03-22
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 计数排序 */
+// 简单实现，无法用于排序对象
+func countingSortNaive(nums: inout [Int]) {
+    // 1. 统计数组最大元素 m
+    let m = nums.max()!
+    // 2. 统计各数字的出现次数
+    // counter[num] 代表 num 的出现次数
+    var counter = Array(repeating: 0, count: m + 1)
+    for num in nums {
+        counter[num] += 1
+    }
+    // 3. 遍历 counter ，将各元素填入原数组 nums
+    var i = 0
+    for num in stride(from: 0, to: m + 1, by: 1) {
+        for _ in stride(from: 0, to: counter[num], by: 1) {
+            nums[i] = num
+            i += 1
+        }
+    }
+}
+
+/* 计数排序 */
+// 完整实现，可排序对象，并且是稳定排序
+func countingSort(nums: inout [Int]) {
+    // 1. 统计数组最大元素 m
+    let m = nums.max()!
+    // 2. 统计各数字的出现次数
+    // counter[num] 代表 num 的出现次数
+    var counter = Array(repeating: 0, count: m + 1)
+    for num in nums {
+        counter[num] += 1
+    }
+    // 3. 求 counter 的前缀和，将“出现次数”转换为“尾索引”
+    // 即 counter[num]-1 是 num 在 res 中最后一次出现的索引
+    for i in stride(from: 0, to: m, by: 1) {
+        counter[i + 1] += counter[i]
+    }
+    // 4. 倒序遍历 nums ，将各元素填入结果数组 res
+    // 初始化数组 res 用于记录结果
+    var res = Array(repeating: 0, count: nums.count)
+    for i in stride(from: nums.count - 1, through: 0, by: -1) {
+        let num = nums[i]
+        res[counter[num] - 1] = num // 将 num 放置到对应索引处
+        counter[num] -= 1 // 令前缀和自减 1 ，得到下次放置 num 的索引
+    }
+    // 使用结果数组 res 覆盖原数组 nums
+    for i in stride(from: 0, to: nums.count, by: 1) {
+        nums[i] = res[i]
+    }
+}
+
+@main
+enum CountingSort {
+    /* Driver Code */
+    static func main() {
+        var nums = [1, 0, 1, 2, 0, 4, 0, 2, 2, 4]
+        countingSortNaive(nums: &nums)
+        print("计数排序（无法排序对象）完成后 nums = \(nums)")
+
+        var nums1 = [1, 0, 1, 2, 0, 4, 0, 2, 2, 4]
+        countingSort(nums: &nums1)
+        print("计数排序完成后 nums1 = \(nums1)")
+    }
+}