Update 0028.实现strStr.md

添加 swift 前缀表两种实现方法

problems/0028.实现strStr.md

@@ -1059,5 +1059,112 @@ func getNext(_ next: inout [Int], needle: [Character]) {
 
 ```
 
+> 前缀表右移
+
+```swift
+func strStr(_ haystack: String, _ needle: String) -> Int {
+
+        let s = Array(haystack), p = Array(needle)
+        guard p.count != 0 else { return 0 }
+
+        var j = 0
+        var next = [Int].init(repeating: 0, count: p.count)
+        getNext(&next, p)
+
+        for i in 0 ..< s.count {
+
+            while j > 0 && s[i] != p[j] {
+                j = next[j]
+            }
+
+            if s[i] == p[j] {
+                j += 1
+            }
+
+            if j == p.count {
+                return i - p.count + 1
+            }
+        }
+
+        return -1
+    }
+
+    // 前缀表后移一位，首位用 -1 填充
+    func getNext(_ next: inout [Int], _ needle: [Character]) {
+
+        guard needle.count > 1 else { return }
+
+        var j = 0
+        next[0] = j
+
+        for i in 1 ..< needle.count-1 {
+
+            while j > 0 && needle[i] != needle[j] {
+                j = next[j-1]
+            }
+
+            if needle[i] == needle[j] {
+                j += 1
+            }
+
+            next[i] = j
+        }
+        next.removeLast()
+        next.insert(-1, at: 0)
+    }
+```
+
+> 前缀表统一不减一
+```swift
+
+func strStr(_ haystack: String, _ needle: String) -> Int {
+
+        let s = Array(haystack), p = Array(needle)
+        guard p.count != 0 else { return 0 }
+
+        var j = 0
+        var next = [Int](repeating: 0, count: needle.count)
+        // KMP
+        getNext(&next, needle: p)
+
+        for i in 0 ..< s.count {
+            while j > 0 && s[i] != p[j] {
+                j = next[j-1]
+            }
+
+            if s[i] == p[j] {
+                j += 1
+            }
+
+            if j == p.count {
+                return i - p.count + 1
+            }
+        }
+        return -1
+    }
+
+    //前缀表
+    func getNext(_ next: inout [Int], needle: [Character]) {
+
+        var j = 0
+        next[0] = j
+
+        for i in 1 ..< needle.count {
+
+            while j>0 && needle[i] != needle[j] {
+                j = next[j-1]
+            }
+
+            if needle[i] == needle[j] {
+                j += 1
+            }
+
+            next[i] = j
+
+        }
+    }
+
+```
+
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