0131.分割回文串：优化排版，补充Swift版本

problems/0131.分割回文串.md

@@ -450,7 +450,8 @@ var partition = function(s) {
 };
 ```
 
-##C
+## C
+
 ```c
 char** path;
 int pathTop;
@@ -546,5 +547,48 @@ char*** partition(char* s, int* returnSize, int** returnColumnSizes){
 }
 ```
 
+## Swift
+
+```swift
+func partition(_ s: String) -> [[String]] {
+    // 把字符串转为字符数组以便于通过索引访问和取子串
+    let s = Array(s)
+    // 使用双指针法判断子串是否回文
+    func isPalindrome(start: Int, end: Int) -> Bool {
+        var start = start, end = end
+        while start < end {
+            if s[start] != s[end] { return false }
+            start += 1
+            end -= 1
+        }
+        return true
+    }
+
+    var result = [[String]]()
+    var path = [String]() // 切割方案
+    func backtracking(startIndex: Int) {
+        // 终止条件，收集结果
+        guard startIndex < s.count else {
+            result.append(path)
+            return
+        }
+
+        for i in startIndex ..< s.count {
+            // 回文则收集，否则跳过
+            if isPalindrome(start: startIndex, end: i) {
+                let substring = String(s[startIndex ... i])
+                path.append(substring)
+            } else {
+                continue
+            }
+            backtracking(startIndex: i + 1) // 寻找下一个起始位置的子串
+            if !path.isEmpty { path.removeLast() } // 回溯
+        }
+    }
+    backtracking(startIndex: 0)
+    return result
+}
+```
+
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