:orphan:

First Unique Character in a String
==================================

.. highlight:: none

Problem
-------
https://leetcode.com/problems/first-unique-character-in-a-string/

Given a string ``s``, find the **first** non-repeating character in it
and return its index. If it **does not** exist, return ``-1``.

 

**Example 1:**

.. container:: example-block

   **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:**

.. container:: example-block

   **Input:** s = "loveleetcode"

   **Output:** 2

**Example 3:**

.. container:: example-block

   **Input:** s = "aabb"

   **Output:** -1

 

**Constraints:**

- ``1 <= s.length <= 10``\ :sup:```5```
- ``s`` consists of only lowercase English letters.

.. highlight:: python

Pattern
-------
Hash Table, String, Queue, Counting

Solution
--------
Use a dictionary to count the number of times each character appears in ``s``.
Iterate through the string again. The first character that has a count of 1 is
the first unique character.

Code
----

.. literalinclude:: ../problems/easy/first-unique-character-in-a-string/first_unique_character_in_a_string__approach_1.py
    :language: python
    :lines: 12-

Test
----
>>> from first_unique_character_in_a_string__approach_1 import firstUniqChar
>>> firstUniqChar('leetcode')
0
>>> firstUniqChar('loveleetcode')
2
>>> firstUniqChar('aabb')
-1

Complexity
----------
| :math:`n` is the length of the input string.
| Time: :math:`O(n)` — 2 passes through the string
| Auxiliary Space: :math:`O(n)` — dictionary stores at most :math:`n` key-value pairs

.. autofunction:: first_unique_character_in_a_string__approach_1.firstUniqChar