First Unique Character

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Given a string s, find the index of the first character that appears exactly once in the entire string. If no such character exists, return -1.

The approach is straightforward: use a hash map to count the frequency of each character in one pass, then walk the string a second time and return the index of the first character whose count equals one. This runs in linear time with constant space regardless of string length.

Return -1 for an empty string or when every character repeats at least twice.

Example 1:

Input: s = "leetcode"
Output: 0
Explanation: 'l' appears only once and is at index 0.

Example 2:

Input: s = "loveleetcode"
Output: 2
Explanation: 'l' and 'o' both repeat. The first non-repeating character is 'v' at index 2.

Example 3:

Input: s = "aabb"
Output: -1
Explanation: Every character appears more than once.

Algorithm Flow

Recommendation Algorithm Flow for First Unique Character - Budibadu

Best Answers

java

import java.util.*;
class Solution {
    public int first_unique_character(String s) {
        Map<Character, Integer> count = new LinkedHashMap<>();
        for (char c : s.toCharArray()) count.merge(c, 1, Integer::sum);
        for (int i = 0; i < s.length(); i++) {
            if (count.get(s.charAt(i)) == 1) return i;
        }
        return -1;
    }
}

javascript

function first_unique_character(...args) {
    const s = args.length === 1 ? args[0] : args.join("");
    const count = {};
    for (const c of s) count[c] = (count[c] || 0) + 1;
    for (let i = 0; i < s.length; i++) {
        if (count[s[i]] === 1) return i;
    }
    return -1;
}

php

<?php
function first_unique_character(...$args) {
    $s = count($args) === 1 ? $args[0] : implode("", $args);
    $count = array_count_values(str_split($s));
    for ($i = 0; $i < strlen($s); $i++) {
        if ($count[$s[$i]] === 1) return $i;
    }
    return -1;
}
?>

python

def first_unique_character(*args) -> int:
    s = args[0] if len(args) == 1 else "".join(args)
    count = {}
    for c in s:
        count[c] = count.get(c, 0) + 1
    for i, c in enumerate(s):
        if count[c] == 1:
            return i
    return -1

rust

use std::collections::HashMap;
fn first_unique_character(s: String) -> i32 {
    let mut count: HashMap<char, i32> = HashMap::new();
    for c in s.chars() {
        *count.entry(c).or_insert(0) += 1;
    }
    for (i, c) in s.chars().enumerate() {
        if *count.get(&c).unwrap_or(&0) == 1 {
            return i as i32;
        }
    }
    -1
}

typescript

function first_unique_character(...args: any[]): number {
    var s: string = args.length === 1 ? args[0] : args.join("");
    var count: { [key: string]: number } = {};
    var i: number;
    for (i = 0; i < s.length; i++) {
        var ch = s.charAt(i);
        count[ch] = (count[ch] || 0) + 1;
    }
    for (i = 0; i < s.length; i++) {
        if (count[s.charAt(i)] === 1) return i;
    }
    return -1;
}

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First Unique Character

Algorithm Flow

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