《Cracking the Coding Interview》——第18章：难题——题目2

2014-04-29 00:59

题目：设计一个洗牌算法，效率尽量快点，必须等概率。

解法：每次随机抽一张牌出来，最后都抽完了，也就洗好了。时间复杂度O(n^2)，请看代码。

代码：

 1 // 18.2 shuffle a deck of 52 cards, it must be perfect random.

 2 #include <cstdio>

 3 #include <cstdlib>

 4 #include <ctime>

 5 #include <vector>

 6 using namespace std;

 7 

 8 void printCards(const vector<int> &cards)

 9 {

10     int i;

11     int n = (int)cards.size();

12     const int col = 8;

13     

14     for (i = 0; i < n; ++i) {

15         printf((i % col == col - 1 ? "%4d\n" : "%4d "), cards[i]);

16     }

17     printf("\n");

18 }

19 

20 void shuffleCards(vector<int> &cards)

21 {

22     vector<int> v;

23     

24     v = cards;

25     int i, j;

26     int n, n0;

27     int idx;

28     

29     n0 = n = (int)cards.size();

30     for (i = 0; i < n0; ++i) {

31         idx = rand() % n;

32         cards[i] = v[idx];

33         --n;

34         for (j = idx; j < n; ++j) {

35             v[j] = v[j + 1];

36         }

37     }

38     

39     v.clear();

40 }

41 

42 int main()

43 {

44     srand((unsigned)time(NULL));

45     vector<int> cards;

46     int i;

47     const int n = 52;

48     

49     cards.resize(n);

50     for (i = 0; i < n; ++i) {

51         cards[i] = i;

52     }

53     

54     shuffleCards(cards);

55     printCards(cards);

56     

57     return 0;

58 }