Merge pull request youngyangyang04#944 from ShrinkLynn/master

[ShrinkLynn] 0028 add swift version

problems/0028.实现strStr.md

@@ -894,7 +894,58 @@ var strStr = function (haystack, needle) {
 };
 ```
 
+Swift 版本
 
+> 前缀表统一减一
+
+```swift
+func strStr(_ haystack: String, _ needle: String) -> Int {
+
+    let s = Array(haystack), p = Array(needle)
+    guard p.count != 0 else { return 0 }
+
+    // 2 pointer
+    var j = -1
+    var next = [Int](repeating: -1, count: needle.count)
+    // KMP
+    getNext(&next, needle: p)
+    for i in 0 ..< s.count {
+        while j >= 0 && s[i] != p[j + 1] {
+            //不匹配之后寻找之前匹配的位置
+            j = next[j]
+        }
+        if s[i] == p[j + 1] {
+            //匹配，双指针同时后移
+            j += 1
+        }
+        if j == (p.count - 1) {
+            //出现匹配字符串
+            return i - p.count + 1
+        }
+    }
+    return -1
+}
+
+//前缀表统一减一
+func getNext(_ next: inout [Int], needle: [Character]) {
+
+    var j: Int = -1
+    next[0] = j
+
+    // i 从 1 开始
+    for i in 1 ..< needle.count {
+        while j >= 0 && needle[i] != needle[j + 1] {
+            j = next[j]
+        }
+        if needle[i] == needle[j + 1] {
+            j += 1;
+        }
+        next[i] = j
+    }
+    print(next)
+}
+
+```
 
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