First Non-Repeating Value

Find the first input value whose final frequency is one.

Algorithm

Canonical input [3, 5, 2, 5, 3, 8, 2] prints 8. The replay uses the same input in every language, so this C# DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.cs

using System;
using System.Collections.Generic;

var arr = new[] {3, 5, 2, 5, 3, 8, 2};
var count = new Dictionary<int, int>();
foreach (var value in arr) {
    count[value] = count.GetValueOrDefault(value) + 1;
}
foreach (var value in arr) {
    if (count[value] == 1) {
        Console.WriteLine(value);
        break;
    }
}

Complexity

Time: O(n) average
Space: O(k) for k distinct values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

two-pass lookup The first pass builds a frequency table. The second pass keeps the original order and stops at the first value with frequency one.