Question

How to find the length of the longest consecutive sequence of an unsorted array of integers in PHP

Answer 1

<?php

function longestConsecutive($nums) {
    if (empty($nums)) {
        return 0;
    }

    // array_flip() returns an array in flip order, 
    // keys from array become values and values from array become keys.
    $numSet = array_flip($nums);
    $longestStreak = 0;
    
    foreach ($nums as $num) {
        // Check if it's the start of a sequence
        // Example: for 680 -> 679 ($num - 1) not exists = Undefined array key = not set =
        // = start at the beginning of a sequence.
        if (!isset($numSet[$num - 1])) {
            $currentNum = $num;
            $currentStreak = 1;

            // Count consecutive numbers
            while (isset($numSet[$currentNum + 1])) {
                $currentNum++;
                $currentStreak++;
            }

            $longestStreak = max($longestStreak, $currentStreak);
        }
    }

    return $longestStreak;
}

$nums = [680, 4, 590, 3, 1, 2];

echo "Length of the longest consecutive sequence: " . longestConsecutive($nums); 



/*
run:

Length of the longest consecutive sequence: 4

*/

 

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