Sandwich Sudoku by Nick Carney $4.99

Sandwich Sudoku comes from Nick Carney. This game was popularized in the spring by Alex Bellos via his puzzle column in The Guardian. 

It's a standard sudoku with one significant difference: the numbers above the grid, and to the side of the grid, are the sums of the numbers that separate the 1 and the 9 in that column or row.

Puzzlers who have played Killer Sudoku (see link below) and Kakuro should be able to jump right in.

The game has only 40 levels, but that's okay! These games have been taking me a long time to solve.  At the moment I've solved 13 levels totaling about 6 hours of fun.  By that measure, $5 is cheap.

This game is fantastic.

My Solving Times
Level   Time
1          14:07
2          31:04
3          26:32
4          26:05
5          11:40
6          44:40
7          23:07
10        26:50
11        31:23
13        28:44
14        23:36
16        21:50
17        24:22

https://apps.apple.com/us/app/sandwich-sudoku/id1476116705

Other Innovative Sudoku Puzzle Apps
Kropk'i Sudoku  This is really a Latin Square with a couple of innovative ideas. It plays fast.
Quindoku Same as Logi5Puzz
Logi5Puzz  Same as NumDom. But NumDom has larger grid sizes.
Spindoku is totally different, but still very cool.
Hi-Lo Sudoku is one of my favorite variations.
Cryptdoku I played it so long ago, but it's cool. Long gone from the App Store.
Stimko Gold - outstanding version of squiggly Sudoku.  Handcrafted puzzles.
Kakooma - interesting for about 3 levels
RollDoku - never even solved 1 level, so hard
Cohabit- played this on my ipod touch - before ipad was invented
FlowDoKu - one of my favorites. Will not delete this.
Sudoku Mine combines Sudoku with Minesweeper
Sudoku Killer  combines Sudoku with Kakuro. I've spent a lot of time on this one. I can do the easy levels in about 10 minutes.
3Doku - a cubic version of Sudoku
Pokergreen Puzzles - a poker version of Sudoku
Star Sudoku
Sudoku Maze
Sudoku Spin
Sudoku Sweeper
Joint Number Place
Futoshiki
SudoCubed
Rotadoku