Скачать или смотреть Advance Practical PHP Big O Notation Non-repeating - video 115W

I want to mentioned the crc32() function.

The crc32() function calculates a 32-bit CRC (cyclic redundancy checksum) for a string. This function can be used to validate data integrity.

Example:

$str = crc32("Hello World!”);

printf("%u",$str);

The Challenge: First Non-Repeating Character

Goal: Given a string (e.g., "aabbcdeeff"), find the first character that only appears once.

Using a Hash Map makes this efficient (O(n)) because we only need to pass through the string twice: once to count, and once to find the first “1".

Hash map approach is the "gold standard" for the non-repeat problem.


The PHP Solution


/**
Finds the first character in a string that does not repeat.
* @param string $string The input string to search.
@return string|null The first unique character, or null if none exist.
*/

function firstUniqueChar($string) {

// Initialize an associative array to store the frequency of each character.
// In PHP, associative arrays function as Hash Maps.

$charCounts = [];
$length = strlen($string);

// -- Pass 1: Frequency Counting --
// We loop through the string to populate our map.

for ($i = 0; $i $length; $i++) {

$char = $string[$i];

// If the character is already a key, increment its count.
// Otherwise, initialize it at 1.

if (i s s e t ($charCounts[$char])) {

$charCounts[$char]++;

} else {

$charCounts[$char] = 1;

}
}

// -- Pass 2: Identification --
// We loop through the string again in its original order.

for ($i = 0; $i $length; $i++) {

// We look up the current character's count in our Hash Map.
// The first one we hit with a value of exactly 1 is our winner.

if ($charCounts[$string[$i]] === 1) {
return $string[$i];
}
}

// If the loop finishes without returning, no unique characters exist.

return null;
}

// Example usage:

echo firstUniqueChar("stress"); // Output: t


Why is this better than the alternative?

Without a Hash Map, you would have to use a nested loop (checking every character against every other character).

Nested Loop: O(n^2) — If the string is 10,000 characters, it takes 100,000,000 operations.

Hash Map: O(n) — If the string is 10,000 characters, it takes roughly 20,000 operations.

Summary of What We've Covered
Direct Mapping: Using keys for instant access.
Frequency Counting: Using values to track occurrences.
Collisions: How PHP handles "overlaps" using chaining.
PHP Array Functions: Using array_filter, array_reduce, and array_map for clean code.

One last thing for your toolkit:

If you ever need to turn a Hash Map back into a simple indexed list of its keys, use array_keys($myMap).

This is super helpful when you've used a map to filter data and just want the final list of "IDs" or "Names."

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https://www.php.net/docs.php

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