Create 37-Sudoku-Solver.java

Author: SharmaTushar1

java/37-Sudoku-Solver.java

@@ -0,0 +1,55 @@
+//this problem is easier than it looks
+//mix of search a 2d matrix and valid sudoku
+
+class Solution {
+    public void solveSudoku(char[][] board) {
+        solve(board);
+    }
+    
+    public boolean solve(char[][] board) {
+        //traverse the complete sudoku and look for empty value
+        for (int i = 0; i<9; i++) {
+            for (int j=0; j<9; j++) {
+                //look for '.' (empty block)
+                if (board[i][j] == '.') {
+                    //try filling all values
+                    for (char c='1'; c<='9'; c++) {
+                        //check if we can put that value at that place
+                        //we will have a function for this which will check all the 
+                        //three conditions for a valid sudoku
+                        if (isValid(board, i, j, c)) {
+                            board[i][j] = c;
+                            //check whether the new board is solvable and just return true if it is
+                            //so, we can just have only one sudoku combination.
+                            //else backtrack, i.e., make it '.' again.
+                            if (solve(board)) 
+                                return true;
+                            else 
+                                board[i][j] = '.';
+                        }
+                    }
+                    //if the board[i][j] is empty and we checked all values and 
+                    //nothing from '0' to '9' can be added then return false
+                    //as this is unsolvable
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+    
+    public boolean isValid (char[][] board, int row, int col, char c) {
+        for (int i = 0; i<9; i++) {
+            //row check
+            if (board[row][i]==c) return false;
+            //col check
+            if (board[i][col]==c) return false;
+            //boxes check 
+            //for this draw a sudoku and see by marking boxes and choosing a random postion
+            //this is hard to explain but try this one
+            //you'll get to the formula by yourself
+            if (board[3*(row/3)+i/3][3*(col/3)+i%3]==c) return false;
+        }
+        return true;
+    }
+}