Knuth's Algorithm X and Dancing Links

1. If the matrix A has no columns, the current partial solution is a valid solution; terminate successfully.
2. Otherwise choose a column c (deterministically).
3. Choose a row r such that Ar, c = 1 (nondeterministically).
4. Include row r in the partial solution.
5. For each column j such that Ar, j = 1,
       for each row i such that Ai, j = 1,
           delete row i from matrix A;
       delete column j from matrix A.
6. Repeat this algorithm recursively on the reduced matrix A.

# remove node x
x.left.right ← x.right;
x.right.left ← x.left;
# restore x
x.left.right ← x;
x.right.left ← x;