Hash Tables

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.

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.

Basic Implementation

basic.cs
Replay: real traced execution (multi-file project)
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;
    }
}

  1. arr ← [3, 5, 2, 5, 3, 8, 2]

    1using System;2using System.Collections.Generic;
    values this step[3, 5, 2, 5, 3, 8, 2]arr

  2. count ← {}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{}count

  3. count ← {3: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{} → {3: 1}count3value

  4. count ← {3: 1, 5: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 1} → {3: 1, 5: 1}count5value

  5. count ← {3: 1, 5: 1, 2: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 1, 5: 1} → {3: 1, 5: 1, 2: 1}count2value

  6. count ← {3: 1, 5: 2, 2: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 1, 5: 1, 2: 1} → {3: 1, 5: 2, 2: 1}count5value

  7. count ← {3: 2, 5: 2, 2: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 1, 5: 2, 2: 1} → {3: 2, 5: 2, 2: 1}count3value

  8. count ← {3: 2, 5: 2, 2: 1, 8: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 2, 5: 2, 2: 1} → {3: 2, 5: 2, 2: 1, 8: 1}count8value

  9. count ← {3: 2, 5: 2, 2: 2, 8: 1}

    4var arr = new[] {3, 5, 2, 5, 3, 8, 2};5var count = new Dictionary<int, int>();6foreach (var value in arr) {
    values this step{3: 2, 5: 2, 2: 1, 8: 1} → {3: 2, 5: 2, 2: 2, 8: 1}count2value

  10. i ← 0, value ← 3, count[value] ← 2, found ← no

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step0i3value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

  11. i ← 1, value ← 5, count[value] ← 2, found ← no

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step1i5value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

  12. i ← 2, value ← 2, count[value] ← 2, found ← no

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step2i2value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

  13. i ← 3, value ← 5, count[value] ← 2, found ← no

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step3i5value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

  14. i ← 4, value ← 3, count[value] ← 2, found ← no

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step4i3value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

  15. i ← 5, value ← 8, count[value] ← 1, found ← yes, result ← 8

    9foreach (var value in arr) {10    if (count[value] == 1) {11        Console.WriteLine(value);
    values this step5i8value1count[value]yesfound8result{3: 2, 5: 2, 2: 2, 8: 1}count

  16. stdout ← 8

    10if (count[value] == 1) {11    Console.WriteLine(value);12    break;
    values this step8stdout8result

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.
  • Dictionary<int, int> stores frequency counts in managed backing storage that grows as needed and is reclaimed by GC. GetValueOrDefault(value) + 1 makes missing-key initialization explicit while integer hashing gives average O(1) lookup/update; collisions are handled inside the dictionary.
  • The second pass reads the managed int[] in original order, so the replay can show the first key whose final count is one.