feat: added NQueens solution in java

Java/algorithms/matrix/NQueens.java

@@ -0,0 +1,51 @@
+package Java.algorithms.matrix;
+
+import java.util.*;
+
+public class NQueens {
+    static List<List<String>> res = new ArrayList<>();
+
+    public static List<List<String>> solveNQueens(int n) {
+        char[][] board = new char[n][n];
+        for (char[] row : board) Arrays.fill(row, '.');
+        solve(board, 0);
+        return res;
+    }
+
+    static void solve(char[][] board, int row) {
+        if (row == board.length) {
+            List<String> temp = new ArrayList<>();
+            for (char[] r : board) temp.add(new String(r));
+            res.add(temp);
+            return;
+        }
+        for (int col = 0; col < board.length; col++) {
+            if (isSafe(board, row, col)) {
+                board[row][col] = 'Q';
+                solve(board, row + 1);
+                board[row][col] = '.';
+            }
+        }
+    }
+
+    static boolean isSafe(char[][] board, int row, int col) {
+        for (int i = 0; i < row; i++)
+            if (board[i][col] == 'Q') return false;
+
+        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--)
+            if (board[i][j] == 'Q') return false;
+
+        for (int i = row - 1, j = col + 1; i >= 0 && j < board.length; i--, j++)
+            if (board[i][j] == 'Q') return false;
+
+        return true;
+    }
+
+    public static void main(String[] args) {
+        List<List<String>> solutions = solveNQueens(4);
+        for (List<String> s : solutions) {
+            for (String row : s) System.out.println(row);
+            System.out.println();
+        }
+    }
+}