added: index of first occurrence

Author: mihirs16

14. Questions/leetcode 28 - index of first occurrence.py

@@ -0,0 +1,26 @@
+# find the index of the first occurrence of a string | leetcode 28 | https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/
+# sliding window to match each character of the haystack with the needle; no slices.
+
+class Solution:
+    def strStr(self, haystack: str, needle: str) -> int:
+        # ----- using regex -----
+        # if needle == '':
+        #     return 0
+        
+        # import re
+        # match = re.search(needle, haystack)
+        # return match.start() if match else -1
+
+        # ----- using sliding windows -----
+        ptrL, ptrR = 0, 0
+        N_needle, N_haystack = len(needle), len(haystack)
+        while ptrR < N_haystack:
+            if haystack[ptrR] == needle[ptrR - ptrL]:
+                ptrR += 1
+                if ptrR - ptrL > N_needle - 1:
+                    return ptrL
+            else:
+                ptrR = ptrL + 1
+                ptrL += 1
+        
+        return -1