Hsien Loong Lee's Sudoku Solver from https://drive.google.com/drive/folders/0B2G2LjIu7WbdfjhaUmVzc1lCR2hUdk5fZllCOHdtbFItbU5qYzdqZGVxdmlnRkJyYVQ4VU0

Sudoku2.cpp

#include "stdio.h"


int InBlock[81], InRow[81], InCol[81];

const int BLANK = 0;
const int ONES = 0x3fe; 	// Binary 1111111110

int Entry[81];	// Records entries 1-9 in the grid, as the corresponding bit set to 1
int Block[9], Row[9], Col[9];	// Each int is a 9-bit array

int SeqPtr = 0;
int Sequence[81];

int Count = 0;
int LevelCount[81];


void SwapSeqEntries(int S1, int S2)
{
     int temp = Sequence[S2];
     Sequence[S2] = Sequence[S1];
     Sequence[S1] = temp;
}


void InitEntry(int i, int j, int val)
{
	 int Square = 9 * i + j;
	 int valbit = 1 << val;
     int SeqPtr2;

     // add suitable checks for data consistency
     
	 Entry[Square] = valbit;
	 Block[InBlock[Square]] &= ~valbit;
	 Col[InCol[Square]] &= ~valbit; // Simpler Col[j] &= ~valbit;
	 Row[InRow[Square]] &= ~valbit; // Simpler Row[i] &= ~valbit;

     SeqPtr2 = SeqPtr;
     while (SeqPtr2 < 81 && Sequence[SeqPtr2] != Square)
           SeqPtr2++ ;

     SwapSeqEntries(SeqPtr, SeqPtr2);
     SeqPtr++;
}


void PrintArray()
{
     int i, j, valbit, val, Square;
     char ch;
     
     Square = 0;

     for (i = 0; i < 9; i++) {
         if (i % 3 == 0) putc('\n', stdout);
         for (j = 0; j < 9; j++) {
             if (j % 3 == 0) putc(' ', stdout);
             valbit = Entry[Square++];
             if (valbit == 0) ch = '-';
             else {
                 for (val = 1; val <= 9; val++) 
                     if (valbit == (1 << val)) {
                        ch = '0' + val;
                        break;
                     }
             }    
             putc(ch,stdout);
         }
         putc ('\n', stdout);
     }
}


void ConsoleInput()
{
     int i, j;
     char InputString[80];

     for (i = 0; i < 9; i++) {
         printf("Row[%d] : ", i + 1);
         scanf("%s", InputString);

         for (j = 0; j < 9; j++) {
             char ch = InputString[j];
             if (ch >= '1' && ch <= '9') 
                InitEntry(i, j, ch - '0');
         }
     }

     PrintArray();
}


void PrintStats()
{
    int i, j, S;

    printf("\nLevel Counts:\n\n");

    S = 0;
    while (LevelCount[S] == 0) S++;

    i = 0;

    while (S < 81) {
          int Seq = Sequence[S];
          printf("(%d, %d):%4d ", Seq / 9 + 1, Seq % 9 + 1, LevelCount[S]);
          if (i++ > 4){
             printf("\n");
             i = 0;
          }
          S++;
    }

    printf("\n\nCount = %d\n", Count);
}


void Succeed()
{
     PrintArray();
     PrintStats();
}


int NextSeq(int S)
{
    int S2, Square, Possibles, BitCount;
    int T, MinBitCount = 100;

    for (T = S; T < 81; T++) {
        Square = Sequence[T];
        Possibles = Block[InBlock[Square]] & Row[InRow[Square]] & Col[InCol[Square]];
        BitCount = 0;
        while (Possibles) {
           Possibles &= ~(Possibles & -Possibles);
           BitCount++;
        }

        if (BitCount < MinBitCount) {
           MinBitCount = BitCount;
           S2 = T;
        }
    }

    return S2;
}


void Place(int S)
{
    LevelCount[S]++;
    Count++;

    if (S >= 81) {
       Succeed();
       return;
    }

    int S2 = NextSeq(S);
    SwapSeqEntries(S, S2);

    int Square = Sequence[S];

    int 	BlockIndex = InBlock[Square],
			RowIndex = InRow[Square],
			ColIndex = InCol[Square];

    int 	Possibles = Block[BlockIndex] & Row[RowIndex] & Col[ColIndex];
    while (Possibles) {
          int valbit = Possibles & (-Possibles); // Lowest 1 bit in Possibles
          Possibles &= ~valbit;
          Entry[Square] = valbit;
          Block[BlockIndex] &= ~valbit;
          Row[RowIndex] &= ~valbit;
          Col[ColIndex] &= ~valbit;
				
          Place(S + 1);

          Entry[Square] = BLANK; // Could be moved out of the loop
          Block[BlockIndex] |= valbit;
          Row[RowIndex] |= valbit;
          Col[ColIndex] |= valbit;
	}

    SwapSeqEntries(S, S2);
}


int main(int argc, char* argv[])
{
	int i, j, Square;

	for (i = 0; i < 9; i++)
		for (j = 0; j < 9; j++) {
			Square = 9 * i + j;
			InRow[Square] = i;
			InCol[Square] = j;
			InBlock[Square] = (i / 3) * 3 + ( j / 3);
		}


	for (Square = 0; Square < 81; Square++) {
        Sequence[Square] = Square;
		Entry[Square] = BLANK;
        LevelCount[Square] = 0;
    }

	for (i = 0; i < 9; i++) 
		Block[i] = Row[i] = Col[i] = ONES;

    ConsoleInput();
    Place(SeqPtr);
    printf("\n\nTotal Count = %d\n", Count);

	return 0;
}

Sudoku2_license.txt

The MIT License (MIT)

Copyright (c) 2015 Lee Hsien Loong

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Sudoku2_sample_run.txt

C:\CPP>sudoku2
Row[1] : ---------
Row[2] : -----3-85
Row[3] : --1-2----
Row[4] : ---5-7---
Row[5] : --4---1--
Row[6] : -9-------
Row[7] : 5------73
Row[8] : --2-1----
Row[9] : ----4---9

 --- --- ---
 --- --3 -85
 --1 -2- ---

 --- 5-7 ---
 --4 --- 1--
 -9- --- ---

 5-- --- -73
 --2 -1- ---
 --- -4- --9

 987 654 321
 246 173 985
 351 928 746

 128 537 694
 634 892 157
 795 461 832

 519 286 473
 472 319 568
 863 745 219

Level Counts:

(3, 9):   1 (8, 9):   2 (4, 9):   4 (5, 9):   8 (6, 9):  15 (1, 9):  23
(3, 8):  33 (8, 8):  46 (4, 8):  79 (1, 8): 119 (5, 8): 182 (6, 8): 250
(9, 8): 287 (4, 7): 347 (6, 7): 478 (6, 5): 588 (6, 1): 732 (6, 3): 828
(6, 4): 862 (6, 6): 895 (4, 5): 795 (4, 3): 761 (5, 5): 843 (7, 5): 829
(2, 5): 616 (1, 5): 594 (2, 7): 543 (2, 3): 565 (7, 3): 551 (2, 4): 577
(3, 6): 565 (3, 4): 590 (1, 4): 572 (1, 6): 595 (7, 4): 612 (5, 4): 576
(7, 6): 476 (7, 7): 389 (7, 2): 262 (4, 2): 184 (2, 2): 140 (2, 1):  95
(5, 6):  56 (8, 7):  34 (8, 6):  18 (3, 1):  13 (8, 1):  10 (3, 7):   7
(3, 2):   8 (1, 7):  10 (5, 1):  10 (5, 2):   6 (8, 2):   4 (8, 4):   2
(9, 1):   1 (1, 1):   1 (9, 2):   1 (9, 3):   1 (9, 4):   1 (4, 1):   1
(9, 6):   1 (9, 7):   1 (1, 3):   1 (1, 2):   1

Count = 17698


Total Count = 24395

C:\CPP>