docs(docs): migrate to array based folder structure

docs/hash-table/387.mdx

@@ -0,0 +1,50 @@
+---
+title: LC-387
+description: First Unique Character in a String
+---
+
+## Examples
+
+| Input            | Output |
+| ---------------- | ------ |
+| `"leetcode"`     | `0`    |
+| `"loveleetcode"` | `2`    |
+| `"aabb"`         | `-1`   |
+
+## Hints
+
+<Accordions type="single">
+  <Accordion title="Constraints">
+    Constraints:
+     • `1 <= s.length <= 10^5`
+     • `s` consists of only lowercase English letters.
+  </Accordion>
+  <Accordion title="Problem Statement">
+    Given a string `s`, find the first non-repeating character in it and return its index.
+    If it does not exist, return `-1`.
+  </Accordion>
+</Accordions>
+
+## Solution
+
+```js
+/**
+ * @param {string} s
+ * @return {number}
+ */
+
+var firstUniqChar = function (s) {
+  // Default for Hammer
+  const freq = {};
+  for (let c of s) freq[c] = (freq[c] || 0) + 1;
+
+  // Problem Specific
+  // Loop the ORIGINAL string — we want an index, so no sorting
+  for (let i = 0; i < s.length; i++) {
+    if (freq[s[i]] === 1) {
+      return i;
+    }
+  }
+  return -1;
+};
+```