feat: add Swift codes for permutations_problem article (#482)

codes/swift/chapter_backtracking/permutations_i.swift

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+/**
+ * File: permutations_i.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 回溯算法：全排列 I */
+func backtrack(state: inout [Int], choices: [Int], selected: inout [Bool], res: inout [[Int]]) {
+    // 当状态长度等于元素数量时，记录解
+    if state.count == choices.count {
+        res.append(state)
+        return
+    }
+    // 遍历所有选择
+    for (i, choice) in choices.enumerated() {
+        // 剪枝：不允许重复选择元素 且 不允许重复选择相等元素
+        if !selected[i] {
+            // 尝试：做出选择，更新状态
+            selected[i] = true
+            state.append(choice)
+            // 进行下一轮选择
+            backtrack(state: &state, choices: choices, selected: &selected, res: &res)
+            // 回退：撤销选择，恢复到之前的状态
+            selected[i] = false
+            state.removeLast()
+        }
+    }
+}
+
+/* 全排列 I */
+func permutationsI(nums: [Int]) -> [[Int]] {
+    var state: [Int] = []
+    var selected = Array(repeating: false, count: nums.count)
+    var res: [[Int]] = []
+    backtrack(state: &state, choices: nums, selected: &selected, res: &res)
+    return res
+}
+
+@main
+enum PermutationsI {
+    /* Driver Code */
+    static func main() {
+        let nums = [1, 2, 3]
+
+        let res = permutationsI(nums: nums)
+
+        print("输入数组 nums = \(nums)")
+        print("所有排列 res = \(res)")
+    }
+}