Singapore prime minister Lee Hsien Loong's Sudoku Solver code runs in 1ms

Singapore's prime minister Lee Hsien Loong showcased his Sudoku Solver C code. You can read his original Facebook post here and another news reporting it here.

I have made some slight modification to adapt it so it can be tested on LeetCode OJ. It passed all 6/6 test cases with a runtime of 1 ms. Pretty impressive for a prime minister, huh?

// Original author: Hsien Loong Lee (http://bit.ly/1zfIGMc)
// Slight modification by @1337c0d3r to adapt to run on LeetCode OJ.
// https://leetcode.com/problems/sudoku-solver/
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];



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(char **board)
{
     int i, j, valbit, val, Square;
     char ch;
     
     Square = 0;

     for (i = 0; i < 9; i++) {
         for (j = 0; j < 9; j++) {
             valbit = Entry[Square++];
             if (valbit == 0) ch = '-';
             else {
                 for (val = 1; val <= 9; val++) 
                     if (valbit == (1 << val)) {
                        ch = '0' + val;
                        break;
                     }
             }    
             board(i)[j] = ch;
         }
     }
}


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, char** board)
{
    if (S >= 81) {
        PrintArray(board);
        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, board);

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

    SwapSeqEntries(S, S2);
}

void solveSudoku(char **board, int m, int n) {
    SeqPtr = 0;
    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;
    }
    
	for (i = 0; i < 9; i++) 
		Block(i) = Row(i) = Col(i) = ONES;
    
    for (int i = 0; i < 9; ++i)
       for (int j = 0; j < 9; ++j) {
           if ('.' != board(i)[j])
                InitEntry(i, j, board(i)[j] - '0');
       }
       
    Place(SeqPtr, board);
}

Source