李显龙：会C++的新加坡总理

40年前，获得一个数学学位后，在父亲的忠告下我继续深造计算机科学。他说这里有未来。后来事实证明，他是对的。我最后的一个项目是数年前用C++写的数独，或许这也说明我过时了。目前我的孩子都在IT圈内，两个毕业于MIT。其中一个拿起一本书给我看，上面写着：“想要更上一层楼，你该学习Haskell。”我想，有一天这会是我退休时的读物。

这是摘自新加坡总理李显龙的一段演讲。

李显龙C++写的数独程序：

#include "stdio.h"int InBlock[81], InRow[81], InCol[81];const int BLANK = 0;const int ONES = 0x3fe; // Binary 1111111110int Entry[81];// Records entries 1-9 in the grid, as the corresponding bit set to 1int Block[9], Row[9], Col[9];// Each int is a 9-bit arrayint 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 consistencyEntry[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 PossiblesPossibles &= ~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 loopBlock[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;}