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First Unique Character in a String

Hashing

15 min

Problem Statement

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

Example

Example 1:

Input: s = "leetcode"

Output: 0

Solution (Hash Map)

A hash map can be used to store the frequency of each character. We can iterate through the string once to build the frequency map, and then a second time to find the first character with a frequency of 1.

Algorithm Steps

Create a hash map to store character counts.
Iterate through the string to populate the hash map with character frequencies.
Iterate through the string again. For each character, check its count in the hash map.
If the count is 1, return the current index.
If the loop completes without finding a unique character, return -1.

String s:

String

Character Counts

Start. First, count character frequencies.

First Unique Character in a String Solution


from collections import Counter

class Solution:
    def firstUniqChar(self, s: str) -> int:
        count = Counter(s)
        for i, c in enumerate(s):
            if count[c] == 1:
                return i
        return -1