Update 028.实现strStr，添加C#版本

problems/0028.实现strStr.md

@@ -1358,6 +1358,72 @@ impl Solution {
 }
 ```
 
+>前缀表统一不减一
+```C#
+public int StrStr(string haystack, string needle)
+{
+    if (string.IsNullOrEmpty(needle))
+        return 0;
+
+    if (needle.Length > haystack.Length || string.IsNullOrEmpty(haystack))
+        return -1;
+
+    return KMP(haystack, needle);
+}
+
+public int KMP(string haystack, string needle)
+{
+    int[] next = GetNext(needle);
+    int i = 0, j = 0;
+    while (i < haystack.Length)
+    {
+        if (haystack[i] == needle[j])
+        {
+            i++;
+            j++;
+        }
+        if (j == needle.Length)
+            return i-j;
+        else if (i < haystack.Length && haystack[i] != needle[j])
+            if (j != 0)
+            {
+                j = next[j - 1];
+            }
+            else
+            {
+                i++;
+            }
+    }
+    return -1;
+}
+
+public int[] GetNext(string needle)
+{
+    int[] next = new int[needle.Length];
+    next[0] = 0;
+    int i = 1, j = 0;
+    while (i < needle.Length)
+    {
+        if (needle[i] == needle[j])
+        {
+            next[i++] = ++j;
+        }
+        else
+        {
+            if (j == 0)
+            {
+                next[i++] = 0;
+            }
+            else
+            {
+                j = next[j - 1];
+            }
+        }
+    }
+    return next;
+}
+```
+
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