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 PHP 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.php
Replay: real traced execution (multi-file project)
<?php
$arr = [3, 5, 2, 5, 3, 8, 2];
$count = [];
foreach ($arr as $value) {
    $count[$value] = ($count[$value] ?? 0) + 1;
}
foreach ($arr as $value) {
    if ($count[$value] === 1) {
        echo $value . PHP_EOL;
        break;
    }
}
?>

  1. arr ← [3, 5, 2, 5, 3, 8, 2]

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step[3, 5, 2, 5, 3, 8, 2]arr

  2. count ← {}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    values this step{}count

  3. count ← {3: 1}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    values this step{} → {3: 1}count3value

  4. count ← {3: 1, 5: 1}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    values this step{3: 1} → {3: 1, 5: 1}count5value

  5. count ← {3: 1, 5: 1, 2: 1}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    values this step{3: 1, 5: 1} → {3: 1, 5: 1, 2: 1}count2value

  6. count ← {3: 1, 5: 2, 2: 1}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    values this step{3: 1, 5: 1, 2: 1} → {3: 1, 5: 2, 2: 1}count5value

  7. count ← {3: 2, 5: 2, 2: 1}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    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}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    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}

    2$arr = [3, 5, 2, 5, 3, 8, 2];3$count = [];4foreach ($arr as $value) {
    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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step0i3value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step1i5value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step2i2value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step3i5value2count[value]nofound{3: 2, 5: 2, 2: 2, 8: 1}count

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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    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

    1<?php2$arr = [3, 5, 2, 5, 3, 8, 2];
    values this step5i8value1count[value]yesfound8result{3: 2, 5: 2, 2: 2, 8: 1}count

  16. stdout ← 8

    8if ($count[$value] === 1) {9    echo $value . PHP_EOL;10    break;
    values this step8stdout8result

Complexity

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

Implementation notes

  • The checked PHP source uses the numeric input array $arr = [3, 5, 2, 5, 3, 8, 2], not a string scan.
  • $count = [] is a PHP associative array used as the frequency table.
  • The first foreach ($arr as $value) walks values in input order and updates counts with $count[$value] = ($count[$value] ?? 0) + 1.
  • The null-coalescing read treats a missing key as 0 before the increment.
  • The trace grows the table as {3: 1}, {3: 1, 5: 1}, {3: 1, 5: 1, 2: 1}, then finishes at {3: 2, 5: 2, 2: 2, 8: 1}.
  • The second foreach scans $arr again instead of iterating the count table, so the result follows original input order.
  • The second pass rejects arr[0]=3, arr[1]=5, arr[2]=2, arr[3]=5, and arr[4]=3 because each has count 2.
  • At arr[5], $value is 8 and $count[$value] === 1, so the code prints 8 and breaks before scanning the final 2.
  • echo $value . PHP_EOL; is the only output path in the checked run.