Feature/chapter dynamic programming swift (#608)

* feat: add Swift codes for intro_to_dynamic_programming article

* feat: add Swift codes for dp_problem_features article

* feat: add Swift codes for dp_solution_pipeline article

* feat: add Swift codes for knapsack_problem article

* feat: add Swift codes for unbounded_knapsack_problem article

* feat: add Swift codes for edit_distance_problem article

codes/swift/chapter_dynamic_programming/climbing_stairs_backtrack.swift

@@ -0,0 +1,42 @@
+/**
+ * File: climbing_stairs_backtrack.swift
+ * Created Time: 2023-07-15
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 回溯 */
+func backtrack(choices: [Int], state: Int, n: Int, res: inout [Int]) {
+    // 当爬到第 n 阶时，方案数量加 1
+    if state == n {
+        res[0] += 1
+    }
+    // 遍历所有选择
+    for choice in choices {
+        // 剪枝：不允许越过第 n 阶
+        if state + choice > n {
+            break
+        }
+        backtrack(choices: choices, state: state + choice, n: n, res: &res)
+    }
+}
+
+/* 爬楼梯：回溯 */
+func climbingStairsBacktrack(n: Int) -> Int {
+    let choices = [1, 2] // 可选择向上爬 1 或 2 阶
+    let state = 0 // 从第 0 阶开始爬
+    var res: [Int] = []
+    res.append(0) // 使用 res[0] 记录方案数量
+    backtrack(choices: choices, state: state, n: n, res: &res)
+    return res[0]
+}
+
+@main
+enum ClimbingStairsBacktrack {
+    /* Driver Code */
+    static func main() {
+        let n = 9
+
+        let res = climbingStairsBacktrack(n: n)
+        print("爬 \(n) 阶楼梯共有 \(res) 种方案")
+    }
+}