0039.组合总和：优化排版，补充Swift版本

problems/0039.组合总和.md

@@ -365,7 +365,7 @@ func backtracking(startIndex,sum,target int,candidates,trcak []int,res *[][]int)
 }
 ```
 
-## JavaScript：
+## JavaScript
 
 ```js
 var combinationSum = function(candidates, target) {
@@ -447,5 +447,32 @@ int** combinationSum(int* candidates, int candidatesSize, int target, int* retur
 }
 ```
 
+## Swift
+
+```swift
+func combinationSum(_ candidates: [Int], _ target: Int) -> [[Int]] {
+    var result = [[Int]]()
+    var path = [Int]()
+    func backtracking(sum: Int, startIndex: Int) {
+        // 终止条件
+        if sum > target { return }
+        if sum == target {
+            result.append(path)
+            return
+        }
+
+        let end = candidates.count
+        guard startIndex < end else { return }
+        for i in startIndex ..< end {
+            path.append(candidates[i]) // 处理
+            backtracking(sum: sum + candidates[i], startIndex: i) // sum这里用新变量完成回溯，i不用+1以重复访问
+            path.removeLast() // 回溯
+        }
+    }
+    backtracking(sum: 0, startIndex: 0)
+    return result
+}
+```
+
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