Added java code and readme updated (#295)

* Added java code and readme updated

* Update README.md

Co-authored-by: Sifat <hossain0338@gmail.com>

Algorithms/Backtracking/Rat in a Maze/README.md

@@ -159,10 +159,143 @@ int main() {
 ```
 <br>
 <h3> Complexity Analysis </h3>
-<p><h> Time Complexity: </h>
+<p><h3> Time Complexity: </h3>
 <br>
 O(9^k) where k is the number of unfilled elements. <br>
 <h>Space Complexity: </h>
 <br>
 O(n*n) where n is the number of rows and columns of the sudoku 2D matrix
 
+	
+## JAVA CODE
+
+```java
+/*package whatever //do not write package name here */
+import java.io.*;
+
+class GFG {
+	static int N = 9;
+
+	// Bitmasks for each row/column/box
+	static int row[] = new int[N], col[] = new int[N],
+			box[] = new int[N];
+	static Boolean seted = false;
+
+	// Utility function to find the box index
+	// of an element at position [i][j] in the grid
+	static int getBox(int i, int j)
+	{
+		return i / 3 * 3 + j / 3;
+	}
+
+	// Utility function to check if a number
+	// is present in the corresponding row/column/box
+	static Boolean isSafe(int i, int j, int number)
+	{
+		return ((row[i] >> number) & 1) == 0
+			&& ((col[j] >> number) & 1) == 0
+			&& ((box[getBox(i, j)] >> number) & 1) == 0;
+	}
+
+	// Utility function to set the initial values of a
+	// Sudoku board (map the values in the bitmasks)
+	static void setInitialValues(int grid[][])
+	{
+		for (int i = 0; i < grid.length; i++) {
+			for (int j = 0; j < grid.length; j++) {
+				row[i] |= 1 << grid[i][j];
+				col[j] |= 1 << grid[i][j];
+				box[getBox(i, j)] |= 1 << grid[i][j];
+			}
+		}
+	}
+
+	/* Takes a partially filled-in grid and attempts
+	to assign values to all unassigned locations in
+	such a way to meet the requirements for
+	Sudoku solution (non-duplication across rows,
+	columns, and boxes) */
+	static Boolean SolveSudoku(int grid[][], int i, int j)
+	{
+		// Set the initial values
+		if (!seted) {
+			seted = true;
+			setInitialValues(grid);
+		}
+
+		if (i == N - 1 && j == N)
+			return true;
+		if (j == N) {
+			j = 0;
+			i++;
+		}
+
+		if (grid[i][j] > 0)
+			return SolveSudoku(grid, i, j + 1);
+
+		for (int nr = 1; nr <= N; nr++) {
+			if (isSafe(i, j, nr)) {
+				/* Assign nr in the
+							current (i, j)
+							position and
+							add nr to each bitmask
+						*/
+				grid[i][j] = nr;
+				row[i] |= 1 << nr;
+				col[j] |= 1 << nr;
+				box[getBox(i, j)] |= 1 << nr;
+
+				if (SolveSudoku(grid, i, j + 1))
+					return true;
+
+				// Remove nr from each bitmask
+				// and search for another possibility
+				row[i] &= ~(1 << nr);
+				col[j] &= ~(1 << nr);
+				box[getBox(i, j)] &= ~(1 << nr);
+			}
+
+			grid[i][j] = 0;
+		}
+
+		return false;
+	}
+
+	// Utility function to print the solved grid
+	static void print(int grid[][])
+	{
+		for (int i = 0; i < grid.length; i++) {
+			for (int j = 0; j < grid[0].length; j++) {
+				System.out.printf("%d ", grid[i][j]);
+			}
+			System.out.println();
+		}
+	}
+
+	// Driver code
+	public static void main(String args[])
+	{
+		// 0 means unassigned cells
+		int grid[][] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
+						{ 5, 2, 0, 0, 0, 0, 0, 0, 0 },
+						{ 0, 8, 7, 0, 0, 0, 0, 3, 1 },
+						{ 0, 0, 3, 0, 1, 0, 0, 8, 0 },
+						{ 9, 0, 0, 8, 6, 3, 0, 0, 5 },
+						{ 0, 5, 0, 0, 9, 0, 6, 0, 0 },
+						{ 1, 3, 0, 0, 0, 0, 2, 5, 0 },
+						{ 0, 0, 0, 0, 0, 0, 0, 7, 4 },
+						{ 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
+
+		if (SolveSudoku(grid, 0, 0))
+			print(grid);
+		else
+			System.out.println("No solution exists");
+	}
+}
+
+
+	
+```
+
+
+