Here's the C++ solution to Hands-on Project: Sudoku Solver.

type=codeblock|id=cpp_sudoku_solver_official_solution|autocreate=cpp|input=5 3 0 0 7 0 0 0 0\n6 0 0 1 9 5 0 0 0\n0 9 8 0 0 0 0 6 0\n8 0 0 0 6 0 0 0 3\n4 0 0 8 0 3 0 0 1\n7 0 0 0 2 0 0 0 6\n0 6 0 0 0 0 2 8 0\n0 0 0 4 1 9 0 0 5\n0 0 0 0 8 0 0 7 9
#include <iostream>
using namespace std;
 
const int UNASSIGNED = 0; // empty cells in the sudoku table
const int N = 9; // sudoku grid size
 
// returns whether or not "number" has been used in particular row
bool usedInRow(int grid[N][N], int row, int number) {
  for (int col = 0; col < N; col++)
    if (grid[row][col] == number)
      return true;
  return false;
}
 
// returns whether or not "number" has been used in particular column
bool usedInCol(int grid[N][N], int col, int number) {
  for (int row = 0; row < N; row++)
    if (grid[row][col] == number)
      return true;
  return false;
}
 
// returns whether or not "number" has been used in particular box
bool usedInBox(int grid[N][N], int row, int col, int number) {
  int boxStartRow = row - row % 3;
  int boxStartCol = col - col % 3;
  for (int ii = 0; ii < 3; ii++)
    for (int jj = 0; jj < 3; jj++)
      if (grid[boxStartRow + ii][boxStartCol + jj] == number)
        return true;
  return false;
}
 
// checks whether or not it is valid to place "number" in the given position
bool isValid(int grid[N][N], int row, int col, int number) {
  return !usedInRow(grid, row, number) &&
       !usedInCol(grid, col, number) &&
       !usedInBox(grid, row, col, number);
}
 
// finds the first unassigned or empty cell in sudoku grid. row and column are passed by reference
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;
}
 
// main sudoku solver function
bool solveSudoku(int grid[N][N]) {
  int row, col;
 
  // if there is no unassigned location, then return true - base case
  if (!findUnassignedLocation(grid, row, col))
    return true;
 
  for (int number = 1; number <= 9; number++) {
    // if safe
    if (isValid(grid, row, col, number)) {
      // assign value
      grid[row][col] = number;
 
      if (solveSudoku(grid))
        return true;
 
      grid[row][col] = UNASSIGNED;
    }
  }
  return false; // this triggers backtracking
}
 
// display the grid
void displayGrid(int grid[N][N]) {
  for (int row = 0; row < N; row++) {
    for (int col = 0; col < N; col++)
      cout << grid[row][col] << " ";
    cout << endl;
  }
}
 
int main() {
  int grid[N][N];
 
  /*
  example:
    0 0 7 9 0 0 6 3 1
    0 0 0 1 0 3 0 4 0
    1 0 0 0 0 5 0 0 9
    6 0 4 8 3 0 0 0 5
    0 0 9 0 0 0 1 0 0
    2 0 0 0 1 7 9 0 4
    5 0 0 3 0 0 0 0 8
    0 9 0 4 0 2 0 0 0
    8 4 6 0 0 1 3 0 0
  */
  cout << "Enter the Sudoku grid (Use 0 for empty cells): " << endl;
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++) {
      cin >> grid[i][j];
    }
  }
  cout << endl;
 
  if (solveSudoku(grid) == true) {
    cout << "Solution: " << endl;
    displayGrid(grid);
  } else {
    cout << "No solution exists. " << endl;
    cout << "Are you sure you entered it correctly?" << endl;
  }
 
  return 0;
}

Sample interaction 1:

Enter the Sudoku grid (Use 0 for empty cells):
5 3 0 0 7 0 0 0 0
6 0 0 1 9 5 0 0 0
0 9 8 0 0 0 0 6 0
8 0 0 0 6 0 0 0 3
4 0 0 8 0 3 0 0 1
7 0 0 0 2 0 0 0 6
0 6 0 0 0 0 2 8 0
0 0 0 4 1 9 0 0 5
0 0 0 0 8 0 0 7 9
 
Solution:
5 3 4 6 7 8 9 1 2
6 7 2 1 9 5 3 4 8
1 9 8 3 4 2 5 6 7
8 5 9 7 6 1 4 2 3
4 2 6 8 5 3 7 9 1
7 1 3 9 2 4 8 5 6
9 6 1 5 3 7 2 8 4
2 8 7 4 1 9 6 3 5
3 4 5 2 8 6 1 7 9

Sample interaction 2:

Enter the Sudoku grid (Use 0 for empty cells):
4 9 0 1 5 7 0 0 0
0 1 8 0 9 0 0 0 0
7 5 0 2 8 4 1 0 6
0 6 0 4 1 5 0 7 0
1 0 0 7 0 0 4 0 0
0 0 0 9 0 8 0 6 1
0 0 7 5 0 0 0 1 3
6 4 0 0 0 0 2 0 0
5 0 1 0 7 0 0 8 0
 
Solution:
4 9 6 1 5 7 8 3 2
2 1 8 3 9 6 7 4 5
7 5 3 2 8 4 1 9 6
9 6 2 4 1 5 3 7 8
1 8 5 7 6 3 4 2 9
3 7 4 9 2 8 5 6 1
8 2 7 5 4 9 6 1 3
6 4 9 8 3 1 2 5 7
5 3 1 6 7 2 9 8 4