Create Sudoku Solver.cpp

Returning is required because we normally store the result in a vector in question of this type, but since it's a bactracking solution, intial and final states of the board will be same, so to avoid we return and stop it from backtracking after solution is made.

Sudoku Solver.cpp

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+/*
+Write a program to solve a Sudoku puzzle by filling the empty cells.
+
+A sudoku solution must satisfy all of the following rules:
+
+Each of the digits 1-9 must occur exactly once in each row.
+Each of the digits 1-9 must occur exactly once in each column.
+Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
+The '.' character indicates empty cells.
+*/
+
+bool isValid(vector<vector<char>>& board, int row, int col, char c) {
+    // row check
+    for(int i = 0; i < 9; i++) 
+		if(board[i][col] == c) 
+			return false;
+	// col check
+    for(int i = 0; i < 9; i++) 
+		if(board[row][i] == c) 
+			return false;
+    // box check
+    int x0 = (row/3) * 3, y0 = (col/3) * 3;
+    for(int i = 0; i < 3; i++) {
+        for(int j = 0; j < 3; j++) {
+            if(board[x0 + i][y0 + j] == c) return false;
+        }
+    }
+    return true;
+}
+
+bool helper(vector<vector<char>> &board, int row, int col) {
+    // done
+    if(row == 9) 
+		return true;
+    // time for next row
+    if(col == 9) 
+		return helper(board, row + 1, 0);
+    // already marked
+    if(board[row][col] != '.') 
+		return helper(board, row, col + 1);
+
+    for(char c = '1'; c <= '9'; c++) {
+        if(isValid(board, row, col, c)) {
+            board[row][col] = c;
+            // without return here, the board reverts to initial state
+            if(helper(board, row, col + 1))
+				return true;
+            board[row][col] = '.';
+        }
+    }
+    return false;
+}
+
+void solveSudoku(vector<vector<char>>& board) {
+    helper(board, 0, 0);
+}