Added NQueens in Java

Java/README.md

@@ -54,6 +54,7 @@ _add list here_
 - [Suduko Solver](cp/SudukoSolver.java)
 - [Ugly Number With Recursion](cp/UglyRecursion.java) 
 - [PDDI Using Recursion](cp/ArmStrongRecursion.java)
+- [NQueens Problem](cp/NQueens.java)
 
 
 ## Cryptography

Java/cp/NQueens.java

@@ -0,0 +1,61 @@
+/**
+N-Queens problem is a famous problem
+The paradigm used to solve the problem is backtracking
+**/
+
+import java.io.*;
+import java.util.*;
+
+public class NQueens {
+
+  public static void solve(boolean[][] board, boolean[] cols, 
+    boolean[] ndiag, boolean[] rdiag, int row, String asf) {
+
+        if(row == board.length) {
+            System.out.println(asf+'.');
+            return;
+        }
+
+        for(int col=0; col<board.length; col++) {
+            if(cols[col] == false && ndiag[row+col] == false && 
+                rdiag[row-col+board.length-1] == false) {
+                    cols[col] = true;
+                    ndiag[row+col] = true;
+                    rdiag[row-col+board.length-1] = true;
+                    board[row][col] = true;
+                    solve(board, cols, ndiag, rdiag, row+1, asf+row+'-'+col+", ");
+                    cols[col] = false;
+                    ndiag[row+col] = false;
+                    rdiag[row-col+board.length-1] = false;
+                    board[row][col] = false;
+                }
+        }
+    }
+
+  public static void main(String[] args) throws Exception {
+    Scanner scn = new Scanner(System.in);
+    int n = scn.nextInt();
+    boolean[][] board = new boolean[n][n];
+    //write your code here
+
+    boolean[] cols = new boolean[n];
+    boolean[] ndiag = new boolean[2*n-1];
+    boolean[] rdiag = new boolean[2*n-1];
+
+    solve(board, cols, ndiag, rdiag, 0, "");
+  }
+
+}
+
+/**
+
+Input :
+4
+Output :
+0-1, 1-3, 2-0, 3-2, .
+0-2, 1-0, 2-3, 3-1, .
+
+Space Complexity : O(n^2)
+Time Complexity : upperbounded by O(n^n)
+
+**/