Missing disk space after repartitioning
Booted to Internet Recovery Mode you have to remove the swap partition and then resize your main volume.

Preparation:


Backup your main volume
Restart to Internet Recovery Mode by pressing alt cmd R at startup. 

The prerequisites are the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated.
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot into a recovery netboot image which usually is loaded from an Apple/Akamai server.

I recommend ethernet because it's more reliable. If you are restricted to WIFI and the boot process fails, just restart your Mac until you succeed booting.

Alternatively you may start from a bootable installer thumb drive (preferably Yosemite or El Capitan) or a thumb drive containing a full system (preferably Yosemite or El Capitan). If you boot to a full system and login as admin you have to prepend sudo to execute some commands like gpt ... or newfs_hfs ...!


Remove the Linux Swap partition


Open in the menubar Utlities->Terminal
Enter diskutil list and gpt -r show /dev/disk0 to get an overview
Remove the Linux partition:

diskutil umountDisk /dev/disk0
gpt remove -i 4 /dev/disk0 



Restore the original size of your main volume


Resize your main volume:

diskutil resizeVolume /dev/disk0s2 750g #or 100%


this should also move your Recovery HD to the end of the disk.
verify the volume with diskutil verifyVolume disk0s2
reboot to your main disk