Question

How to find a common element in all rows of a given matrix with sorted rows in PHP

Answer 1

function findCommonElementInMatrixRows(array $matrix): int {
    $rows = count($matrix);
    if ($rows === 0) return -1;

    $cols = count($matrix[0]);
    $map = [];

    foreach ($matrix as $row) {
        $map[$row[0]] = ($map[$row[0]] ?? 0) + 1;
        for ($j = 1; $j < $cols; $j++) {
            if ($row[$j] !== $row[$j - 1]) {
                $val = $row[$j];
                $map[$val] = ($map[$val] ?? 0) + 1;
            }
        }
    }

    foreach ($map as $key => $count) {
        if ($count === $rows) {
            return $key;
        }
    }

    return -1;
}

$matrix = [
    [1, 2, 3, 5, 36],
    [4, 5, 7, 9, 10],
    [5, 6, 8, 9, 18],
    [1, 3, 5, 8, 24]
];

$result = findCommonElementInMatrixRows($matrix);

if ($result !== -1) {
    echo "Common element in all rows: $result\n";
} else {
    echo "No common element found in all rows.\n";
}


/*
run:

Common element in all rows: 5

*/

 

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