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Merge pull request #211 from Karthik08-github/master
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once plz check the update code for n queeens program implementation
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@IEEE-IIITSricity
IEEE-IIITSricity committed Oct 4, 2020
2 parents 75f2e6e + 20adfe6 commit f1c6fda
Showing 1 changed file with 89 additions and 162 deletions.
251 changes: 89 additions & 162 deletions Backtracking/N-Queen.java
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Original file line number Diff line number Diff line change
@@ -1,178 +1,105 @@
/**
* CREATED BY ROSHAN SINGH
* CREATED BY T S T KARTHIK
*
* 12:32 AM 08/04/20
* 13:20 AM 04/10/20
*/
import java.io.*;
import java.util.*;
class Main{
static class Reader // for number only problems
{
final private int BUFFER_SIZE = 1 << 16;
private DataInputStream din;
private byte[] buffer;
private int bufferPointer, bytesRead;
public Reader()
{
din = new DataInputStream(System.in);
buffer = new byte[BUFFER_SIZE];
bufferPointer = bytesRead = 0;
}
public Reader(String file_name) throws IOException
{
din = new DataInputStream(new FileInputStream(file_name));
buffer = new byte[BUFFER_SIZE];
bufferPointer = bytesRead = 0;
}
public String readLine() throws IOException
{
byte[] buf = new byte[64]; // line length
int cnt = 0, c;
while ((c = read()) != -1)
{
if (c == '\n')
break;
buf[cnt++] = (byte) c;
}
return new String(buf, 0, cnt);
}
public int nextInt() throws IOException
{
int ret = 0;
byte c = read();
while (c <= ' ')
c = read();
boolean neg = (c == '-');
if (neg)
c = read();
do
{
ret = ret * 10 + c - '0';
} while ((c = read()) >= '0' && c <= '9');

if (neg)
return -ret;
return ret;
}
public long nextLong() throws IOException
{
long ret = 0;
byte c = read();
while (c <= ' ')
c = read();
boolean neg = (c == '-');
if (neg)
c = read();
do {
ret = ret * 10 + c - '0';
}
while ((c = read()) >= '0' && c <= '9');
if (neg)
return -ret;
return ret;
}
public double nextDouble() throws IOException
{
double ret = 0, div = 1;
byte c = read();
while (c <= ' ')
c = read();
boolean neg = (c == '-');
if (neg)
c = read();
do {
ret = ret * 10 + c - '0';
}
while ((c = read()) >= '0' && c <= '9');
if (c == '.')
{
while ((c = read()) >= '0' && c <= '9')
{
ret += (c - '0') / (div *= 10);
}
}
if (neg)
return -ret;
return ret;
}
private void fillBuffer() throws IOException
{
bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
if (bytesRead == -1)
buffer[0] = -1;
}
private byte read() throws IOException
{
if (bufferPointer == bytesRead)
fillBuffer();
return buffer[bufferPointer++];
}
public void close() throws IOException
{
if (din == null)
return;
din.close();
}
/**
* N-Queens is the problem to plane N chess queens into a NxN board without
* in a way that no queen threats another one.
* This implementation uses recursion and backtracking to find the solution.
*/
public class NQueens {

/**
* Tries to place N queens into an NxN chess board
*
* @param n number of queens
* @return a solution array, where the indexes are the columns of the board, and the value is the row that
* the queen should be placed in that column.
*/
public int[] placeQueens(int n) {

int[] solution = new int[n];

placeQueens(0, solution);
return solution;
}

static void n_queen(int[] arr,int level,int n,LinkedList<LinkedList<Integer>> result){
if(level == n){
// System.out.println(Arrays.toString);
if(arr[level-1] != 0){
LinkedList<Integer> ll = new LinkedList<>();
for (int i = 0; i < n; i++) {
ll.add((int)(Math.log(arr[i])/Math.log(2) +1 ));
/**
* Recursive method that tries to place the queen on column 'column', and then passes the current partial solution
* to it again, until all columns are solved.
* @param column the column that should be solved.
* @param solution the partial solution until now.
* @return true if it solved the placement problem, false otherwise.
*/
private boolean placeQueens(int column, int[] solution) {
// base case, all columns are ok
if (column == solution.length) {
return true;
}
for (int row = 0; row < solution.length; row++) {
solution[column] = row;
if (noThreats(solution, column)) {
if (placeQueens(column+1, solution)) {
return true;
}
result.add(ll);
}else{
return;
}
return;
}
int mask = (1<<n)-1;
for(int i = 0;i<level;i++){
mask &= ~arr[i];
mask &= ~(arr[i]>>(level-i));
mask &= ~((arr[i]<<(level-i))&((1<<n)-1));
}
// System.out.println(Integer.toBinaryString(mask)+" "+level);
// System.out.println(Integer.toBinaryString(arr[level]));
solution[column] = -1;
return false;
}

for (int i = 0; i < n; i++) {
if(((mask>>i)&1) == 1){
arr[level] = (1<<i);
// System.out.println(Integer.toBinaryString(arr[level])+" "+level);
n_queen(arr,level+1,n,result);
arr[level] = 0;
/**
* This methods check the queens placement for column 'column'
* @param solution current partial solution
* @param column column index that should be verified against all previous columns
* @return true if there are no threats to 'column' queen, false otherwise.
*/
private boolean noThreats(int[] solution, int column) {
for (int i = 0; i < column; i++) {
if (solution[i] == solution[column]) {
return false;
}
}
}
public static void main(String[] args) throws IOException {
Reader s = new Reader();
// Reader s = new Reader("INPUT");
OutputStream outputStream = System.out;
// OutputStream outputStream = new FileOutputStream("OUTPUT");
PrintWriter o = new PrintWriter(outputStream);
// start
int t = s.nextInt();
while (t-->0){
int n = s.nextInt();
LinkedList<LinkedList<Integer>> result = new LinkedList<>();
int[] a = new int[n];
n_queen(a,0,n,result);
if(result.size() == 0){
o.println("-1");
continue;
int diff = column - i;
if (solution[i] + diff == solution[column]) {
return false;
}
for (LinkedList<Integer> i : result){
o.print("[");
for (Integer j : i) o.print(j+" ");
o.print("] ");
if (solution[i] - diff == solution[column]) {
return false;
}
o.println("");
}
// end
o.close();
return true;
}

/**
* Prints a solution to System.out with ascii drawing
* @param solution
*/
public static void printSolution(int[] solution) {
for (int i = 0; i < solution.length; i++) {
System.out.print(" _");
}
System.out.println();
for (int i = 0; i < solution.length; i++) {
for (int j = 0; j < solution.length; j++) {
System.out.print("|");
if (i == solution[j]) {
System.out.print("@");
} else {
System.out.print("_");
}
}
System.out.println("|");
}
}
}

public static void main(String[] args) {
NQueens nQueens = new NQueens();
printSolution(nQueens.placeQueens(9));
}

}



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