NQueens 

The nqueens problem is a classic combinatorial problem.
It is required to place
Note that not every permutation is a solution. A permutation generator can be turned into an nqueens solver by adding tests to check diagonals.
This program has been modified in a simple way from the permutation generator. The identifiers have been changed to reflect chess terminology. It is assumed that a partial solution, board[1..col1], is safe and an attempt is made to place a queen in column col. If this is impossible the routine returns. If it is possible and the new queen threatens no previous queen then the extended solution is safe and a recursive call is made to place the remaining queens. The process succeeds when all queens are placed. For all n greater than or equal to four there are solutions and in fact more than one solution for each value of n. A search for solutions to a combinatorial problem carried out in this way is known as backtracking. This comes from the way that partial solutions are extended, backing up at blind alleys where the constraints cannot be satisfied, until one or more complete solutions are found. Notes
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