— Alex Reinhart and Christopher Genovese

Backtracking is a general strategy for solving constraint satisfaction problems: we have a bunch of constraints on possible solutions, and we must try possible solutions until we find one that satisfies all the constraints.

There are many problems that fit this mold. Some examples:

Room assignments
We have k classes that have to be fit into n rooms. Each class can only fit into rooms large enough for it. There are a limited number of time slots available for each room. We must assign classes to rooms so all classes get a time slot and fit in their room.
Graph coloring
Color each node in a graph with one of k colors, such that no node is connected to another node of the same color. (Includes map coloring.)
Logic programming
Specify a problem as a set of logical expressions or rules and find values of variables which make the expressions true.

One prominent example is also a popular puzzle.

Sudoku #

One canonical example where backtracking algorithms shine is Sudoku, which you may already be familiar with:


(example from Wikipedia)

A Sudoku grid is a 9 × 9 grid, divided into separate 3 × 3 blocks. It starts with numbers in some of the cells, as in the example above. Your goal is to fill in the empty cells with digits from 1 to 9, so that the same digit does not appear twice in any row, column, or 3 × 3 block. You can’t change numbers already provided in the grid above.

For example, the above grid can be solved as follows:


Sudoku is a popular puzzle to solve by hand. We can, however, take all the fun out of it by writing a program to solve it.

One possible method is to simply try every combination of numbers and see which combinations meet the rules. However, this grid has 51 open cells, meaning the number of possible ways we can fill those cells is \(9^{51} = 4.64 \times 10^{48}\). That would be stunningly inefficient. Search time would be exponential in the number of cells.

Of course, nearly all of those combinations are invalid: multiple cells break the rules. Suppose, instead of enumerating complete grids, we start with partial solutions – say, just filling in one cell – and gradually fill in more and more cells, until we either (a) solve the puzzle or (b) realize we are stuck. If we get stuck, we don’t bother filling in the rest of the grid and trying all the possibilities for it – we backtrack, erasing the previous thing we did and trying a different option.

[Tree diagram]

The key idea is that backtracking prunes the tree: when we backtrack, we throw away large numbers of candidate solutions, meaning we never have to look at any of them. This dramatically cuts down the search time.

Implementation #

We need several pieces to implement this.

  1. A data structure to represent the Sudoku grid as it is currently filled in – the current partial solution.
  2. A way to determine which moves are possible, e.g. which cells we can fill and with which numbers.
  3. A function to choose which cell we should fill next.
  4. A function to choose which digit, of possibly several possibilities, we should choose to fill a cell.
  5. The core backtracking algorithm.

Our choices for 3 and 4 matter quite a lot: if we choose correctly, we can rapidly reduce the search space by eliminating impossible grids; if we choose unwisely, we may spend a lot of time exploring a portion of the space that has no possible solutions. We will need to design our algorithm so we can easily try different choices, to determine which performs the best.

For simplicity, we’ll combine 1 and 2 into one: our representation for the grid will be a matrix, where every grid cell contains a vector of all currently possible values for that cell. (In Python, we’ll use a dictionary of sets.)

This leads to a few core functions:

  • assign assigns a value to a single cell, eliminating that possible value from all peer cells
  • eliminate eliminates a possible value from a cell. For consistency, if it results in a cell having only one possible value, that value is eliminated from all peer cells. This is constraint propagation.

Finally, we have solve_sudoku:

solve_sudoku <- function(grid, next_cell, order_choices) {
    if (identical(grid, FALSE)) {
        ## Must have died earlier

    if (solved(grid)) {
        ## We have solved the grid.

    #### INSERT CODE HERE ####

Requirements #

This activity is posted as the sudoku homework assignment in the problem bank.