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apps/docs/content/hash-table/387-first-unique-character-in-a-string.mdx

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+---
+title: '387. First Unique Character in a String'
+description: Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1
+sidebar:
+  label: 'First Unique Character in a String'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Hash Table</Badge>
+
+### Example 1:
+- Input: `s = "leetcode"`
+- Output: `0`
+- Explanation: The character `'l'` at index `0` is the first character that does not occur at any other index.
+
+### Example 2:
+- Input: `s = "loveleetcode"`
+- Output: `2`
+
+### Example 3:
+- Input: `s = "aabb"`
+- Output: `-1`
+
+### Constraints:
+
+- `1 <= s.length <= 10^5`
+- `s` consists of only lowercase English letters.
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["firstUniqChar(s)"]) --> F["Pass 1 — freq: count every char of s"]
+  F --> L{"Pass 2 — more i in 0..len-1?"}
+  L -- no --> E(["return -1"])
+  L -- yes --> Q{"freq[s[i]] == 1?"}
+  Q -- yes --> R(["return i — leftmost wins because the scan is in order"])
+  Q -- no --> L
+```
+
+## Solution
+
+```js
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var firstUniqChar = function (s) {
+  const freq = {};
+  for (let c of s) freq[c] = (freq[c] || 0) + 1;
+
+  for (let i = 0; i < s.length; i++) {
+    if (freq[s[i]] === 1) {
+      return i;
+    }
+  }
+  return -1;
+};
+```
+
+## Explanation
+
+<YouTube id="rBENYgWy3xU" title="First Unique Character in a String - Leetcode 387 - Python" />