0040.组合总和II：优化排版，补充Swift版本

problems/0040.组合总和II.md

@@ -381,7 +381,7 @@ class Solution:
             sum_ -= candidates[i]       # 回溯，为了下一轮for loop
 ```
 
-## Go：
+## Go
 主要在于如何在回溯中去重
 
 ```go
@@ -424,7 +424,7 @@ func backtracking(startIndex,sum,target int,candidates,trcak []int,res *[][]int,
 }
 ```
 
-## javaScript： 
+## javaScript
 
 ```js
 /**
@@ -550,5 +550,37 @@ int** combinationSum2(int* candidates, int candidatesSize, int target, int* retu
 }
 ```
 
+## Swift
+
+```swift
+func combinationSum2(_ candidates: [Int], _ target: Int) -> [[Int]] {
+    // 为了方便去重复，先对集合排序
+    let candidates = candidates.sorted()
+    var result = [[Int]]()
+    var path = [Int]()
+    func backtracking(sum: Int, startIndex: Int) {
+        // 终止条件
+        if sum == target {
+            result.append(path)
+            return
+        }
+
+        let end = candidates.count
+        guard startIndex < end else { return }
+        for i in startIndex ..< end {
+            if i > startIndex, candidates[i] == candidates[i - 1] { continue } // 去重复
+            let sum = sum + candidates[i] // 使用局部变量隐藏回溯
+            if sum > target { continue } // 剪枝
+
+            path.append(candidates[i]) // 处理
+            backtracking(sum: sum, startIndex: i + 1) // i+1避免重复访问
+            path.removeLast() // 回溯
+        }
+    }
+    backtracking(sum: 0, startIndex: 0)
+    return result
+}
+```
+
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