Find the first character in a string that appears exactly once and return its index.
Given a string s, identify the first character that occurs exactly once in the string and return its index. If no such character exists, return -1.
A character is considered unique if it appears one time in the entire string. The answer should be the smallest index among all unique characters.
Input Format
- A single string
s. smay contain lowercase letters, uppercase letters, digits, or other visible characters depending on the platform variant.
Output Format
- Return the index of the first non-repeating character.
- If every character repeats, return
-1.
Constraints
- The string length is at least 1.
- A linear-time solution is expected.
- Use extra space only as needed to track character counts.
Example 1
Input
s = "leetcode"
Output
0
Explanation
The character 'l' appears once and is the first unique character, so the answer is index 0.
Example 2
Input
s = "loveleetcode"
Output
2
Explanation
Characters 'l' and 'o' repeat, but 'v' appears exactly once at index 2, which is the first unique character.
Show 1 more example
Example 3
Input
s = "aabb"
Output
-1
Explanation
Every character appears more than once, so there is no unique character.
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