Merge pull request sanscript-tech#330 from Sapna2001/sudoku

Sudoku solver in Java

Author: tejan-singh

Java/Sudoku_Solver/Readme.md

@@ -0,0 +1,18 @@
+# Sudoku Solver
+This is a program to solve a sudoku. Instead of the blank spaces, 0 has to be inputed.
+
+## Sample Input
+![input](https://user-images.githubusercontent.com/56690856/98670547-d74b0e00-2378-11eb-9fe2-44e124d1cef5.png)
+
+## Sample Output
+![output](https://user-images.githubusercontent.com/56690856/98670752-2bee8900-2379-11eb-837f-1b613c5137ca.png)
+
+## Demo
+![demo](https://user-images.githubusercontent.com/56690856/98670816-49235780-2379-11eb-9245-2d6b6aa02ef8.png)
+
+## Time Complexity
+O(9^(n*n))
+
+## Space Complexity
+O(n*n)
+

Java/Sudoku_Solver/SudokuSolver.java

@@ -0,0 +1,92 @@
+package sudoku;
+
+import java.util.*;
+
+public class SudokuSolver {
+	
+	// N=9 as there will be 9 rows and 9 colums
+	static int N = 9; 
+	
+	static boolean solveSudoku(int grid[][], int row, int col) {
+		// if we cross the last cell then we return true to avoid further back-tracking
+		if (row == N - 1 && col == N) 
+            return true; 
+		// if we reach the last column then we move to the next row after setting col as 0
+		if (col == N)  
+        { 
+            row++; 
+            col = 0; 
+        } 
+		// check whether the cell is 0 and if it is not 0 then we move to the next column for further processing
+		if (grid[row][col] != 0) 
+            return solveSudoku(grid, row, col + 1); 
+		
+		// check if a no from 1 to 0=9 can be placed in a given row and column
+		for(int no=1;no<10;no++) {
+			if (isSafe(grid,row,col,no)) {
+				// assigning no to the cell assuming it be correct
+				grid[row][col] = no;
+				
+			// then go for the next column assuming the no assigned is correct	
+			if (solveSudoku(grid,row,col+1))
+				return true;	
+			}
+			// if the assumption is wrong assign 0 to the cell
+			grid[row][col] = 0; 
+		}
+		return false;
+	}
+	
+	static boolean isSafe(int grid[][], int row, int col,int num) {
+		// return false if the given num is found in the column
+		for (int i=0;i<8;i++) {
+			if (grid[i][col]==num)
+				return false;
+		}
+		
+		// return false if the given num is found in the row
+		for (int i=0;i<8;i++) {
+			if (grid[row][i]==num)
+				return false;
+		}
+		// check if num is present in the 3*3 matrix
+		int startRow = row -(row%3);
+		int startColumn = col -(col%3);
+		for(int i=0;i<3;i++) {
+			for(int j=0;j<3;j++) {
+				if (grid[i+startRow][j+startColumn]==num)
+					return false;
+			}
+		}
+		return true;
+	}
+	
+	// display the solved sudoku
+	static void print(int[][] grid) {
+		System.out.println("Solved Sudoku");
+		for(int i=0;i<N;i++) {
+			for(int j=0;j<N;j++)
+				System.out.print(grid[i][j]+" ");
+			System.out.println();
+		}
+	}
+ 
+	public static void main(String[] args) {
+		// TODO Auto-generated method stub
+		Scanner sc = new Scanner(System.in);
+		int[][] grid = new int[N][N];
+		System.out.println("Enter the Sudoku to be solved");
+		// enter the matrix
+		for(int i=0; i<N;i++) {
+			for(int j=0;j<N;j++) {
+				grid[i][j]=sc.nextInt();
+			}
+		}
+		// check if solution exists
+		if(solveSudoku(grid,0,0))
+			print(grid);
+		else
+			System.out.println("Solution does not exist");
+	}
+
+}