[String] Add a solution to Implement strStr

String/StrStr.swift

@@ -6,7 +6,7 @@
 
 class StrStr {
     func strStr(_ haystack: String, _ needle: String) -> Int {
-        let hChars = Array(haystack.characters), nChars = Array(needle.characters)
+        let hChars = Array(haystack), nChars = Array(needle)
         let hLen = hChars.count, nLen = nChars.count
 
         guard hLen >= nLen else {
@@ -31,4 +31,32 @@ class StrStr {
 
         return -1
     }
-}
+}
+
+
+
+class StrStrWithSubstring {
+    func strStr(_ haystack: String, _ needle: String) -> Int {
+        guard needle.count > 0 else {
+            return 0
+        }
+
+        guard haystack.count >= needle.count else {
+            return -1
+        }
+
+        let shiftIndex = needle.count - 1
+        let maxLowerIndex = haystack.count - needle.count
+
+        for lowerIndex in 0...maxLowerIndex {
+            let lowerBound: String.Index = haystack.index(haystack.startIndex, offsetBy: lowerIndex)
+            let upperBound: String.Index = haystack.index(haystack.startIndex, offsetBy: lowerIndex + shiftIndex)
+
+            if haystack[lowerBound...upperBound] == needle {
+                return lowerIndex
+            }
+        }
+
+        return -1
+    }
+}