add problem 28

28. Implement strStr()/problem.md

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+# 28. Implement strStr()
+
+Implement strStr().
+
+Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
+
+**Clarification:**
+
+What should we return when needle is an empty string? This is a great question to ask during an interview.
+
+For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().
+
+**Example 1:**
+
+```
+Input: haystack = "hello", needle = "ll"
+Output: 2
+```
+
+**Example 2:**
+
+```
+Input: haystack = "aaaaa", needle = "bba"
+Output: -1
+```
+
+**Example 3:**
+
+```
+Input: haystack = "", needle = ""
+Output: 0
+```
+
+**Constraints:**
+
+* 0 <= haystack.length, needle.length <= 5 * 104
+* haystack and needle consist of only lower-case English characters.

28. Implement strStr()/solution.js

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+/**
+ * @param {string} haystack
+ * @param {string} needle
+ * @return {number}
+ */
+ var strStr = function(haystack, needle) {
+    if (needle === "") return 0;
+    if (needle.length > haystack.length) return -1;
+
+    let haystackPointer = 0;
+    let needlePointer = 0;
+
+    while (haystackPointer < haystack.length && needlePointer < needle.length) {
+        if (haystack[haystackPointer] === needle[needlePointer]) {
+            haystackPointer++;
+            needlePointer++;
+        } else {
+            haystackPointer = haystackPointer + 1 - needlePointer;
+            needlePointer = 0;
+        }
+    }
+
+    if (needlePointer === needle.length) return haystackPointer - needle.length;
+
+    return -1;
+};