Description
Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.
Examples
Input:
s = "leetcode"Output:
0Explanation:
Counting character frequencies: l=1, e=2, t=1, c=1, o=1, d=1. Scanning left to right, 'l' at index 0 is the first character with count 1.
Input:
s = "loveleetcode"Output:
2Explanation:
Character 'l' appears twice, 'o' appears twice, but 'v' at index 2 appears only once. Scanning from left, 'v' is the first with frequency 1.
Input:
s = "aabb"Output:
-1Explanation:
Both 'a' and 'b' appear exactly twice in the string. Since every character repeats, there is no unique character to return.
Constraints
- •
1 ≤ s.length ≤ 10⁵ - •
s consists of only lowercase English letters.