0077.组合优化：补充Swift版本

problems/0077.组合优化.md

@@ -294,5 +294,35 @@ int** combine(int n, int k, int* returnSize, int** returnColumnSizes){
 }
 ```
 
+Swift：
+
+```swift
+func combine(_ n: Int, _ k: Int) -> [[Int]] {
+    var path = [Int]()
+    var result = [[Int]]()
+    func backtracking(_ n: Int, _ k: Int, _ startIndex: Int) {
+        // 结束条件，并收集结果
+        if path.count == k {
+            result.append(path)
+            return
+        }
+
+        // 单层逻辑
+        // let end = n
+        // 剪枝优化
+        let end = n - (k - path.count) + 1
+        guard startIndex <= end else { return }
+        for i in startIndex ... end {
+            path.append(i) // 处理结点
+            backtracking(n, k, i + 1) // 递归
+            path.removeLast() // 回溯
+        }
+    }
+
+    backtracking(n, k, 1)
+    return result
+}
+```
+
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