二进制相加

Given two binary strings, return their sum (also a binary string).

For example, a = “11” b = “1” Return “100”.

主要处理好进位的问题，PHP实现代码如下：

function BinaryAdd($num1, $num2) {    $last_index_1 = strlen($num1) - 1;    $last_index_2 = strlen($num2) - 1;    // 是否进位    $flag = 0;    $binary = '';    while ($last_index_1 >= 0 || $last_index_2 >= 0) {        $binary1 = $last_index_1 < 0 ? 0 : $num1[$last_index_1];        $binary2 = $last_index_2 < 0 ? 0 : $num2[$last_index_2];        $sum = $binary1 + $binary2 + $flag;        // 判断是否需要进位 和为2或者3时需要进位        switch ($sum) {            case 2:                $flag = 1;                $binary = '0' . $binary;                break;            case 3:                $flag = 1;                $binary = '1' . $binary;                break;            default:                $binary = $sum . $binary;                $flag = 0;                break;        }        $last_index_1--;        $last_index_2--;    }    // 如果有进位，则首位补1    if ($flag == 1) $binary = '1' . $binary;    return $binary;}echo BinaryAdd('11', '1');