Why a Sudoku-solving program looks smart: Recursion, backtracking, and the idea of 'trying from the most likely failure points'

I was thinking about a program to solve Sudoku.You fill a 9x9 grid with numbers from 1 to 9. The same number must not be ...
I Programmer

Forward and backward recursion

The first list of instructions is obeyed on the way up the spiral i.e. as the instances of the function are being built and the second list is obeyed on the way back down the spiral as the instances ...