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CSE143X Sample Program handout #21
Sample Executions of Queens.java
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This program solves the classic '8 queens'
problem, placing queens on a chessboard so
that no two queens threaten each other.
What size board do you want to use? 8
One solution is as follows:
Q - - - - - - -
- - - - - - Q -
- - - - Q - - -
- - - - - - - Q
- Q - - - - - -
- - - Q - - - -
- - - - - Q - -
- - Q - - - - -
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This program solves the classic '8 queens'
problem, placing queens on a chessboard so
that no two queens threaten each other.
What size board do you want to use? 2
No solution.
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This program solves the classic '8 queens'
problem, placing queens on a chessboard so
that no two queens threaten each other.
What size board do you want to use? 4
One solution is as follows:
- - Q -
Q - - -
- - - Q
- Q - -
Program file Queens.java
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// This program solves the classic "8 queens" problem using recursive
// backtracking.
import java.util.*;
public class Queens {
public static void main(String[] args) {
giveIntro();
Scanner console = new Scanner(System.in);
System.out.print("What size board do you want to use? ");
int size = console.nextInt();
System.out.println();
Board b = new Board(size);
solve(b);
}
// post: explains program to user
public static void giveIntro() {
System.out.println("This program solves the classic '8 queens'");
System.out.println("problem, placing queens on a chessboard so");
System.out.println("that no two queens threaten each other.");
System.out.println();
}
// pre : queens have been safely placed in columns 1 through (col - 1)
// post: recursively searchs the board for a solution starting at col,
// returning true iff such a solution occurs and storing the
// solution in board if it exists
private static boolean explore(Board b, int col) {
if (col > b.size())
return true;
else {
for (int row = 1; row <= b.size(); row++)
if (b.safe(row, col)) {
b.place(row, col);
if (explore(b, col + 1))
return true;
b.remove(row, col);
}
return false;
}
}
// post: searches for a solution to the 8 queens problem with this
// board, reporting result
public static void solve(Board solution) {
if (!explore(solution, 1))
System.out.println("No solution.");
else {
System.out.println("One solution is as follows:");
solution.print();
}
}
}
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