Sudoku - 9x9 app for iPhone and iPad

A sudoku is a grid of nine by 9 squares. The grid has 9 rows, 9 columns and 9 3x3 square regions.

The goal of Sudoku is to fill a 9 x 9 square grid with numbers, so that each row, each column
and 3x3 section contains the numbers 1 to 9. At the start of the game, the 9x9 grid will have squares already filled.
Your task will be to use the logic to fill in the missing numbers and complete the grid.
Note that a displacement is incorrect if:
- Each line contains several of the same number from 1 to 9
- Each column contains several of the same number from 1 to 9
- Each grid of 3x3 cells contains several of the same number from 1 to 9

Solving a sudoku requires logic. No mathematical calculation is necessary to find the solution.