From 72dc12b20068e53dee86f8fa18019c1adac3633f Mon Sep 17 00:00:00 2001
From: Alerika <alessiaiaconantonio.ai@gmail.com>
Date: Mon, 14 Oct 2019 18:46:46 +0200
Subject: [PATCH] add Sudoku

---
 README.md            |   3 +
 Sudokuc++/sudoku.c++ | 131 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 Sudokuc++/sudoku.c++

diff --git a/README.md b/README.md
index e086270e..cd4131fe 100644
--- a/README.md
+++ b/README.md
@@ -21,3 +21,6 @@ check some of these resources to help you out to implement basic c# algorithms:
 - [Coin Greedy Algorithm](https://www.csharpstar.com/csharp-coin-change-problem-greedy-algorithm/)
 
 If you think something is missing inside this Readme.md, make a pull request and change it :)
+
+## Add
+- Sudoku in c++
diff --git a/Sudokuc++/sudoku.c++ b/Sudokuc++/sudoku.c++
new file mode 100644
index 00000000..5fc156f7
--- /dev/null
+++ b/Sudokuc++/sudoku.c++
@@ -0,0 +1,131 @@
+#include <stdio.h>
+
+// UNASSIGNED is used for empty cells in sudoku grid
+#define UNASSIGNED 0
+
+// N is used for size of Sudoku grid. Size will be NxN
+#define N 9
+
+// This function finds an entry in grid that is still unassigned
+bool FindUnassignedLocation(int grid[N][N], int &row, int &col);
+
+// Checks whether it will be legal to assign num to the given row,col
+bool isSafe(int grid[N][N], int row, int col, int num);
+
+/* 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) */
+bool SolveSudoku(int grid[N][N])
+{
+	int row, col;
+
+	// If there is no unassigned location, we are done
+	if (!FindUnassignedLocation(grid, row, col))
+		return true; // success!
+
+	// consider digits 1 to 9
+	for (int num = 1; num <= 9; num++)
+	{
+		// if looks promising
+		if (isSafe(grid, row, col, num))
+		{
+			// make tentative assignment
+			grid[row][col] = num;
+
+			// return, if success, yay!
+			if (SolveSudoku(grid))
+				return true;
+
+			// failure, unmake & try again
+			grid[row][col] = UNASSIGNED;
+		}
+	}
+	return false; // this triggers backtracking
+}
+
+/* Searches the grid to find an entry that is still unassigned. If
+   found, the reference parameters row, col will be set the location
+   that is unassigned, and true is returned. If no unassigned entries
+   remain, false is returned. */
+bool FindUnassignedLocation(int grid[N][N], int &row, int &col)
+{
+	for (row = 0; row < N; row++)
+		for (col = 0; col < N; col++)
+			if (grid[row][col] == UNASSIGNED)
+				return true;
+	return false;
+}
+
+/* Returns a boolean which indicates whether any assigned entry
+   in the specified row matches the given number. */
+bool UsedInRow(int grid[N][N], int row, int num)
+{
+	for (int col = 0; col < N; col++)
+		if (grid[row][col] == num)
+			return true;
+	return false;
+}
+
+/* Returns a boolean which indicates whether any assigned entry
+   in the specified column matches the given number. */
+bool UsedInCol(int grid[N][N], int col, int num)
+{
+	for (int row = 0; row < N; row++)
+		if (grid[row][col] == num)
+			return true;
+	return false;
+}
+
+/* Returns a boolean which indicates whether any assigned entry
+   within the specified 3x3 box matches the given number. */
+bool UsedInBox(int grid[N][N], int boxStartRow, int boxStartCol, int num)
+{
+	for (int row = 0; row < 3; row++)
+		for (int col = 0; col < 3; col++)
+			if (grid[row+boxStartRow][col+boxStartCol] == num)
+				return true;
+	return false;
+}
+
+/* Returns a boolean which indicates whether it will be legal to assign
+   num to the given row,col location. */
+bool isSafe(int grid[N][N], int row, int col, int num)
+{
+	/* Check if 'num' is not already placed in current row,
+	   current column and current 3x3 box */
+	return !UsedInRow(grid, row, num) &&
+		!UsedInCol(grid, col, num) &&
+		!UsedInBox(grid, row - row%3 , col - col%3, num);
+}
+
+/* A utility function to print grid  */
+void printGrid(int grid[N][N])
+{
+	for (int row = 0; row < N; row++)
+	{
+		for (int col = 0; col < N; col++)
+			printf("%2d", grid[row][col]);
+		printf("\n");
+	}
+}
+
+// Test program
+int main()
+{
+	// 0 means unassigned cells
+	int grid[N][N] = {{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) == true)
+		printGrid(grid);
+	else
+		printf("No solution exists");
+
+	return 0;
+}