Classical Sudoku Master Puzzle

Tap an empty cell and select a number to fill it. The board is a 9x9 grid divided into nine 3x3 boxes. Some numbers are pre-filled.
Use logic to place numbers 1-9 so that each digit appears only once in each row, column, and 3x3 box.
Four difficulty levels are offered. Higher difficulty means fewer given numbers and greater challenge, with increased satisfaction upon solving.