PHP程序员必备：掌握查找缺失数字的技巧

function findMissingNumberLinear($arr) {
    $n = count($arr) + 1;
    for ($i = 1; $i <= $n; $i++) {
        if (!in_array($i, $arr)) {
            echo "缺失的数字为：".$i;
            break;
        }
    }
}

function findMissingNumberMath($arr) {
    $n = count($arr) + 1;
    $sum = ($n * ($n + 1)) / 2;
    $arr_sum = array_sum($arr);
    $missing_number = $sum - $arr_sum;
    echo "缺失的数字为：".$missing_number;
}

function findMissingNumberBit($arr) {
    $n = count($arr) + 1;
    $xor1 = 0;
    $xor2 = 0;
    for ($i = 1; $i <= $n; $i++) {
        $xor1 ^= $i;
    }
    foreach ($arr as $num) {
        $xor2 ^= $num;
    }
    $missing_number = $xor1 ^ $xor2;
    echo "缺失的数字为：".$missing_number;
}