feat: add Swift codes for subset_sum_problem article (#584)

codes/swift/Package.swift

@@ -68,6 +68,9 @@ let package = Package(
         .executable(name: "preorder_traversal_iii_template", targets: ["preorder_traversal_iii_template"]),
         .executable(name: "permutations_i", targets: ["permutations_i"]),
         .executable(name: "permutations_ii", targets: ["permutations_ii"]),
+        .executable(name: "subset_sum_i_naive", targets: ["subset_sum_i_naive"]),
+        .executable(name: "subset_sum_i", targets: ["subset_sum_i"]),
+        .executable(name: "subset_sum_ii", targets: ["subset_sum_ii"]),
         .executable(name: "n_queens", targets: ["n_queens"]),
     ],
     targets: [
@@ -137,6 +140,9 @@ let package = Package(
         .executableTarget(name: "preorder_traversal_iii_template", dependencies: ["utils"], path: "chapter_backtracking", sources: ["preorder_traversal_iii_template.swift"]),
         .executableTarget(name: "permutations_i", path: "chapter_backtracking", sources: ["permutations_i.swift"]),
         .executableTarget(name: "permutations_ii", path: "chapter_backtracking", sources: ["permutations_ii.swift"]),
+        .executableTarget(name: "subset_sum_i_naive", path: "chapter_backtracking", sources: ["subset_sum_i_naive.swift"]),
+        .executableTarget(name: "subset_sum_i", path: "chapter_backtracking", sources: ["subset_sum_i.swift"]),
+        .executableTarget(name: "subset_sum_ii", path: "chapter_backtracking", sources: ["subset_sum_ii.swift"]),
         .executableTarget(name: "n_queens", path: "chapter_backtracking", sources: ["n_queens.swift"]),
     ]
 )

codes/swift/chapter_backtracking/subset_sum_i.swift

@@ -0,0 +1,53 @@
+/**
+ * File: subset_sum_i.swift
+ * Created Time: 2023-07-02
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 回溯算法：子集和 I */
+func backtrack(state: inout [Int], target: Int, choices: [Int], start: Int, res: inout [[Int]]) {
+    // 子集和等于 target 时，记录解
+    if target == 0 {
+        res.append(state)
+        return
+    }
+    // 遍历所有选择
+    // 剪枝二：从 start 开始遍历，避免生成重复子集
+    for i in stride(from: start, to: choices.count, by: 1) {
+        // 剪枝一：若子集和超过 target ，则直接结束循环
+        // 这是因为数组已排序，后边元素更大，子集和一定超过 target
+        if target - choices[i] < 0 {
+            break
+        }
+        // 尝试：做出选择，更新 target, start
+        state.append(choices[i])
+        // 进行下一轮选择
+        backtrack(state: &state, target: target - choices[i], choices: choices, start: i, res: &res)
+        // 回退：撤销选择，恢复到之前的状态
+        state.removeLast()
+    }
+}
+
+/* 求解子集和 I */
+func subsetSumI(nums: [Int], target: Int) -> [[Int]] {
+    var state: [Int] = [] // 状态（子集）
+    let nums = nums.sorted() // 对 nums 进行排序
+    let start = 0 // 遍历起始点
+    var res: [[Int]] = [] // 结果列表（子集列表）
+    backtrack(state: &state, target: target, choices: nums, start: start, res: &res)
+    return res
+}
+
+@main
+enum SubsetSumI {
+    /* Driver Code */
+    static func main() {
+        let nums = [3, 4, 5]
+        let target = 9
+
+        let res = subsetSumI(nums: nums, target: target)
+
+        print("输入数组 nums = \(nums), target = \(target)")
+        print("所有和等于 \(target) 的子集 res = \(res)")
+    }
+}

codes/swift/chapter_backtracking/subset_sum_i_naive.swift

@@ -0,0 +1,51 @@
+/**
+ * File: subset_sum_i_naive.swift
+ * Created Time: 2023-07-02
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 回溯算法：子集和 I */
+func backtrack(state: inout [Int], target: Int, total: Int, choices: [Int], res: inout [[Int]]) {
+    // 子集和等于 target 时，记录解
+    if total == target {
+        res.append(state)
+        return
+    }
+    // 遍历所有选择
+    for i in stride(from: 0, to: choices.count, by: 1) {
+        // 剪枝：若子集和超过 target ，则跳过该选择
+        if total + choices[i] > target {
+            continue
+        }
+        // 尝试：做出选择，更新元素和 total
+        state.append(choices[i])
+        // 进行下一轮选择
+        backtrack(state: &state, target: target, total: total + choices[i], choices: choices, res: &res)
+        // 回退：撤销选择，恢复到之前的状态
+        state.removeLast()
+    }
+}
+
+/* 求解子集和 I（包含重复子集） */
+func subsetSumINaive(nums: [Int], target: Int) -> [[Int]] {
+    var state: [Int] = [] // 状态（子集）
+    let total = 0 // 子集和
+    var res: [[Int]] = [] // 结果列表（子集列表）
+    backtrack(state: &state, target: target, total: total, choices: nums, res: &res)
+    return res
+}
+
+@main
+enum SubsetSumINaive {
+    /* Driver Code */
+    static func main() {
+        let nums = [3, 4, 5]
+        let target = 9
+
+        let res = subsetSumINaive(nums: nums, target: target)
+
+        print("输入数组 nums = \(nums), target = \(target)")
+        print("所有和等于 \(target) 的子集 res = \(res)")
+        print("请注意，该方法输出的结果包含重复集合")
+    }
+}

codes/swift/chapter_backtracking/subset_sum_ii.swift

@@ -0,0 +1,58 @@
+/**
+ * File: subset_sum_ii.swift
+ * Created Time: 2023-07-02
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 回溯算法：子集和 II */
+func backtrack(state: inout [Int], target: Int, choices: [Int], start: Int, res: inout [[Int]]) {
+    // 子集和等于 target 时，记录解
+    if target == 0 {
+        res.append(state)
+        return
+    }
+    // 遍历所有选择
+    // 剪枝二：从 start 开始遍历，避免生成重复子集
+    // 剪枝三：从 start 开始遍历，避免重复选择同一元素
+    for i in stride(from: start, to: choices.count, by: 1) {
+        // 剪枝一：若子集和超过 target ，则直接结束循环
+        // 这是因为数组已排序，后边元素更大，子集和一定超过 target
+        if target - choices[i] < 0 {
+            break
+        }
+        // 剪枝四：如果该元素与左边元素相等，说明该搜索分支重复，直接跳过
+        if i > start, choices[i] == choices[i - 1] {
+            continue
+        }
+        // 尝试：做出选择，更新 target, start
+        state.append(choices[i])
+        // 进行下一轮选择
+        backtrack(state: &state, target: target - choices[i], choices: choices, start: i + 1, res: &res)
+        // 回退：撤销选择，恢复到之前的状态
+        state.removeLast()
+    }
+}
+
+/* 求解子集和 II */
+func subsetSumII(nums: [Int], target: Int) -> [[Int]] {
+    var state: [Int] = [] // 状态（子集）
+    let nums = nums.sorted() // 对 nums 进行排序
+    let start = 0 // 遍历起始点
+    var res: [[Int]] = [] // 结果列表（子集列表）
+    backtrack(state: &state, target: target, choices: nums, start: start, res: &res)
+    return res
+}
+
+@main
+enum SubsetSumII {
+    /* Driver Code */
+    static func main() {
+        let nums = [4, 4, 5]
+        let target = 9
+
+        let res = subsetSumII(nums: nums, target: target)
+
+        print("输入数组 nums = \(nums), target = \(target)")
+        print("所有和等于 \(target) 的子集 res = \(res)")
+    }
+}