First Unique Character in a String

Approach

Use a two-pass frequency counting strategy:

1. Count each character occurrence in s.
2. Scan s from left to right.
3. Return the first index where the character frequency is 1.
4. If no such character is found, return -1.

Why this works:

• A character is unique exactly when its total count is 1.
• The second pass preserves left-to-right order, so the first match is the required answer.

Implementation

export function firstUniqChar(s: string): number {
    const frequency = new Map<string, number>();
 
    for (const char of s) {
        frequency.set(char, (frequency.get(char) ?? 0) + 1);
    }
 
    for (let i = 0; i < s.length; i++) {
        const char = s[i];
        if (frequency.get(char) === 1) return i;
    }
 
    return -1;
}

Complexity

• Time O(n): one pass to count and one pass to find the first unique index.
• Space O(1): bounded alphabet ('a' to 'z') means constant extra storage.

Test Coverage

The test suite validates:

• LeetCode baseline examples
• Empty and single-character inputs
• Unique character at start, middle, and end
• All-duplicated strings returning -1
• Large deterministic lowercase inputs