LeetCode 387: First Unique Character in a String Solution

Master LeetCode problem 387 (First Unique Character in a String), a easy challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

387. First Unique Character in a String

Easy

Hash Table String Queue Counting

Problem Explanation

Explanation

To solve this problem, we can iterate through the input string s and store the frequency of each character in a HashMap. Then we iterate through s again to find the first character with a frequency of 1 in the HashMap, which will be our answer.

Time complexity: O(n) where n is the length of the input string s
Space complexity: O(1) since the size of the HashMap is limited by the number of lowercase English letters

Solution Code

class Solution {
    public int firstUniqChar(String s) {
        Map<Character, Integer> freq = new HashMap<>();
        
        for (char c : s.toCharArray()) {
            freq.put(c, freq.getOrDefault(c, 0) + 1);
        }
        
        for (int i = 0; i < s.length(); i++) {
            if (freq.get(s.charAt(i)) == 1) {
                return i;
            }
        }
        
        return -1;
    }
}

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