A unique character is one that appears only once in a string.
Given a string consisting of lowercase English letters only, return the index of the first occurrence of a unique character in the string using 1-based indexing.
If the string does not contain any unique character, return -1.
Example:s = "statistics"
The unique characters are [a, c], and a occurs first.
Using 1-based indexing, its index is 3.
Another Example:
Input: "hackthegame"
Unique characters: [c, k, t, g, m]
The character c occurs first at index 3.
The task is to find the first non-repeating character in a lowercase string and return its 1-based index.
Use a frequency count:
- Count occurrences of each character.
- Scan the string again; return the index of the first character with frequency 1.
- If none exists, return -1.
Runs in O(n) time and uses O(1) extra space.
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