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

codes/swift/chapter_backtracking/subset_sum_i_naive.swift

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+/**
+ * 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")
+    }
+}