In order to solve advanced sudoku puzzles, begin by filling in the easy numbers. You can then proceed to use the so-called X-Wing Technique and the Swordfish Technique.
Begin by filling in the easy numbers, which is also known as the Sole Candidate Strategy. This involves identifying any squares that can only house one possible number. Since each column, row and section on the puzzle must contain all the numbers between one and nine, some cells can immediately be filled with one obvious number.
Once all possible numbers have been placed in this manner, move on to the Unique Candidate Strategy. This technique is similar to the Sole Candidate Strategy, but involves identifying specific areas that can only hold a specific number in relation to the placement of the other instances of the same number throughout the puzzle.
After filling in as many numbers as possible, move on to advanced techniques. The X-Wing Strategy involves identifying two rows that have only two squares that can potentially hold a single number. The strategy uses the process of elimination to rule out the squares where that number can be placed based on the logical relationships within the grid. The Swordfish Technique is a more advanced version of the X-Wing, as it involves three rows instead of two. However, it uses the same logical process of elimination to fill the cells.