Added the NQueen problem for the issue #136

JAVA/NQueen.java

@@ -0,0 +1,85 @@
+import java.util.Scanner;
+
+public class NQueen {
+    final int N;
+
+    public NQueen(int N) {
+        this.N = N;
+    }
+
+    /* This is the function to print the solution for the problem*/
+    void printSolution(int board[][]) {
+        for (int i = 0; i < N; i++) {
+            for (int j = 0; j < N; j++) {
+                System.out.print(board[i][j] == 1 ? "Q " : ". ");
+            }
+            System.out.println();
+        }
+        System.out.println();
+    }
+
+    /*It checks if a queen can be placed on board[row][col] */
+    boolean isSafe(int board[][], int row, int col) {
+        // Check this row on left side
+        for (int i = 0; i < col; i++) {
+            if (board[row][i] == 1) {
+                return false;
+            }
+        }
+
+        // It checks upper diagonal on left side
+        for (int i = row, j = col; i >= 0 && j >= 0; i--, j--) {
+            if (board[i][j] == 1) {
+                return false;
+            }
+        }
+
+        //It checks lower diagonal on left side
+        for (int i = row, j = col; j >= 0 && i < N; i++, j--) {
+            if (board[i][j] == 1) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /*It is used to solve the N-Queen problem using backtracking */
+    boolean solveNQUtil(int board[][], int col) {
+        if (col >= N) {
+            printSolution(board);
+            return true;
+        }
+
+        boolean res = false;
+        for (int i = 0; i < N; i++) {
+            if (isSafe(board, i, col)) {
+                board[i][col] = 1;
+
+                //It is recursively placing rest of the queens
+                res = solveNQUtil(board, col + 1) || res;
+
+                //It is used for backtracking and removing queen
+                board[i][col] = 0;
+            }
+        }
+
+        return res;
+    }
+
+    /* This function will solve the N-Queen problem and prints all solutions */
+    void solveNQ() {
+        int board[][] = new int[N][N];
+
+        if (!solveNQUtil(board, 0)) {
+            System.out.println("Solution does not exist");
+        }
+    }
+    public static void main(String[] args) {
+        Scanner sc=new Scanner(System.in);
+        System.out.println("Enter the value of N");
+        int N = sc.nextInt(); //Here the input is taken from the user
+        NQueen Queen = new NQueen(N);
+        Queen.solveNQ();
+    }
+}