solve problem

leet-code/28-find-the-index-of-the-first-occurrence-in-a-string/README.md

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+# 28-find-the-index-of-the-first-occurrence-in-a-string
+
+Given two strings `needle` and `haystack`, return the index of the first occurrence of `needle` in `haystack`, or -1 if `needle` is not part of `haystack`.
+
+---
+
+### Example 1:
+
+**Input:**  
+`haystack = "sadbutsad"`  
+`needle = "sad"`
+
+**Output:**  
+`0`
+
+**Explanation:**  
+"sad" occurs at index 0 and 6.  
+The first occurrence is at index 0, so we return 0.
+
+---
+
+### Example 2:
+
+**Input:**  
+`haystack = "leetcode"`  
+`needle = "leeto"`
+
+**Output:**  
+`-1`
+
+**Explanation:**  
+"leeto" did not occur in "leetcode", so we return -1.
+
+---
+
+### Constraints:
+
+- `1 <= haystack.length, needle.length <= 10^4`
+- `haystack` and `needle` consist of only lowercase English characters.

leet-code/28-find-the-index-of-the-first-occurrence-in-a-string/index.js

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+/**
+ * @param {string} haystack
+ * @param {string} needle
+ * @return {number}
+ */
+var strStr = function(haystack, needle) {
+  // needles can't exceed the length of the haystack
+  if (haystack.length < needle.length) return -1;
+
+  // stop searching once the remaining characters aren't at least as long as needle
+  for (let i = 0; i <= haystack.length - needle.length; i++) {
+    // the needle is found
+    if (haystack.substring(i, i + needle.length) === needle) return i
+  }
+  // fallback to not found
+  return -1
+};