Sudoku - methods of solving

Sudoku methods:

step 1: check the obvious ones: go through each number in the top 3 squares vertically, and check, how often they appear, if maybe in one row there is only one spot left to be filled, etc. Do this for the 3 middle squares and the 3 bottom squares, then the same horizontally.

step 2: take a pencil and go through the free spot one by one and try all numbers 1-9 and write down the ones that could possibly go in there in the top corner. Start with the most promising ones, where you already have lots of numbers around that obviously can't appear there.

step 3: check line by line horizontally and vertically, and square by square, which penciled numbers only appear once per line or square. The number only appearing once of course goes into that spot.

step 4: Lets take a look at the top 3 squares as an example. Lets assume, the number 1 only is already in the first square, in the top row, but not in the next two squares. Lets also assume, in the middle square, the middle row is already filled. That means, the number 1 can't be in the first row, because thats where it is in the first square, and not in the second row, because all the spots are taken, therefore it must be in the third row. Because the number 1 ist in the first square in the top row, and in the second square in the third row, as established, it must be in the middle row in the third square.
This method works for all numbers, as long as you have a number only once in three squares, and in one of the three squares one row is full.