This is a simple command line program written in C++ to solve and check solutions for sudoku puzzles. I programmed it for my Hacker School application. The sudoku is represented as an array of arrays.

sudoku_checker.cpp

//
//  main.cpp
//  sudoku_checker
//
//  Created by Corey McMahon on 3/22/13.
//  Copyright (c) 2013 Corey McMahon. All rights reserved.
//

#include <iostream>
using namespace std;

void printit(int s[][9]){
    for(int i = 0; i<9; i++){
        if(i==3 || i==6){
            cout << "-----------------------------------------" << endl;
        }
        for(int j = 0; j<9; j++){
            if(j!=8){
                if(j==3 || j==6){
                    cout << "|   " << s[i][j] << "   ";
                }else{
                    cout << s[i][j] << "   ";
                }
            }
            else{
                cout << s[i][j] << endl;
            }
        }
    }
}

void insertValue(int val, int row, int col, int s[][9]){
    if(row > 9 || row<1){
        cout << "\nERROR: INVALID ROW NUMBER\n\n";
    }else if(col>9 || col<1){
        cout << "\nERROR: INVALID COLUMN NUMBER\n\n";
    }else if(val>9 || val<1){
        cout << "\nERROR: INVALID VALUE\n\n";
    }else{
        s[(row-1)][(col-1)] = val;
    }
}

// Looks for duplicates in an array of length 9
// returns true if there ARE duplicates
bool checkDuplicates(int arr[]){
    bool b = false;
    for(int i=0; i<9; i++){
        if(arr[i]==0)
            ;
        else{
            for(int j=0; j<9; j++){
                if(i==j)
                    ;
                else{
                    if(arr[i]==arr[j]) b = true;
                }
                
            }
        }
    }
    return b;
    
}

// returns false if solution is NOT valid, true if it is.
bool checkValidity(int s[][9]){
    // Check rows
    for(int i=0; i<9; i++){
        int rowarray[9];
        for(int j=0; j<9; j++){
            rowarray[j] = s[i][j];
        }
        if(checkDuplicates(rowarray)){
            return false;
        }
    }
    
    // Check columns
    for(int i=0; i<9; i++){
        int colarray[9];
        for(int j=0; j<9; j++){
            colarray[j] = s[j][i];
        }
        if(checkDuplicates(colarray)){
            return false;
        }
    }
    
    // Check squares
    
    for(int x=0; x<3; x++){
        for(int i=0; i<3; i++){
            int sqrarray[9];
            for(int j=0; j<3; j++){
                for(int h=0; h<3; h++){
                    sqrarray[(3*j)+h] = s[j+(3*x)][(3*i)+h];
                }
            }
            if(checkDuplicates(sqrarray)){
                return false;
            }
        }
    }
    
    
    return true;
    
}

int main(int argc, const char * argv[])
{
    // sudoku[row][column]
    int sudoku[9][9];
    
    // Initialize the sudoku matrix
    for(int i = 0; i<9; i++){
        for(int j = 0; j<9; j++){
            sudoku[i][j] = 0;
        }
    }
    
    cout << "YOUR SUDOKU:\n";
    printit(sudoku);
    cout << "What would you like to do?\n(1) Enter value\n(2) Check validity\n(3) Quit\n";
    int inputval = 0;
    cin >> inputval;
    
    while(inputval != 3){
        if(inputval ==1){
            int value, row, column;
            cout << "What value you would like to enter? ";
            cin >> value;
            cout << "In what row would you like to place this value? ";
            cin >> row;
            cout << "In what column would you like to place this value? ";
            cin >> column;
            insertValue(value, row, column, sudoku);
            
            cout << "YOUR SUDOKU:\n";
            printit(sudoku);
            cout << "What would you like to do?\n(1) Enter value\n(2) Check validity\n(3) Quit\n";
            cin >> inputval;
        }else if(inputval==2){
            
            if(checkValidity(sudoku)==false){
                cout << "Your solution is invalid\n";
            }else{
                cout << "Your solution is valid\n";
            }
            
            cout << "What would you like to do?\n(1) Enter value\n(2) Check validity\n(3) Quit\n";
            
            cin >> inputval;
            
        }else{
            cout << "Improper input value\n\n" << "What would you like to do?\n(1) Enter value\n(2) Check validity\n(3) Quit\n";
            cin >> inputval;
        }
    }
    return 0;
}