Add Binary

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

For example,
a = "11"
b = "1"
Return "100".

char* addBinary(char* a, char* b) {
    int asize = strlen(a), bsize = strlen(b);
     int up = 0, rsize = asize;
    char *result;
    if (asize < bsize)
        rsize = bsize;
    result = malloc(rsize + 2);
    result[++rsize] = '\0';
    while (asize && bsize)
    {
        if (a[asize - 1] + b[bsize - 1] - 2 * '0' + up > 1)
        {
            result[rsize - 1] = a[asize - 1] + b[bsize - 1] - '0' + up - 2;
            up = 1;
        }
        else
        {
            result[rsize - 1] = a[asize - 1] + b[bsize - 1] - '0' + up;
            up = 0;
        }
        asize--;
        bsize--;
        rsize--;
    }
    while (asize)
    {
        result[--rsize] = a[--asize] + up;
        if (result[rsize] - '0'> 1)
        {
            result[rsize] = '0';
            up = 1;
        }
        else
            up = 0;
    }
    while (bsize)
    {
        result[--rsize] = b[--bsize] + up;
        if (result[rsize] - '0'> 1)
        {
            result[rsize] = '0';
            up = 1;
        }
        else
            up = 0;
    }
    if (up)
    {
        result[rsize - 1] = up + '0';
        return result;
    }
    else
        return result + 1;
}

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