《Cracking the Coding Interview》——第18章：难题——题目3

2014-04-29 01:02

题目：从m个整数里随机选出n个整数，要求等概率。

解法：和洗牌的算法类似，每次随机抽出一个数，抽n次即可。时间复杂度O(m * n)，空间复杂度O(m)。

代码：

 1 // 18.3 pick m integers randomly from an array of n integer.

 2 #include <cstdio>

 3 #include <cstdlib>

 4 #include <ctime>

 5 #include <vector>

 6 using namespace std;

 7 

 8 void randomSubset(const vector<int> &src, vector<int> &dst, int m)

 9 {

10     vector<int> v;

11     int i, j;

12     int idx;

13     int n;

14     

15     v = src;

16     n = (int)v.size();

17     dst.resize(m);

18     for (i = 0; i < m; ++i) {

19         idx = rand() % n;

20         dst[i] = v[idx];

21         --n;

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

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

24         }

25     }

26     v.clear();

27 }

28 

29 int main()

30 {

31     int i;

32     int n, m;

33     vector<int> src;

34     vector<int> dst;

35     int cc;

36     

37 

38     cc = 0;

39     while (scanf("%d%d", &n, &m) == 2) {

40         ++cc;

41         srand((unsigned)(time(NULL) * cc));

42         src.resize(n);

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

44             src[i] = i;

45         }

46         randomSubset(src, dst, m);

47         for (i = 0; i < m; ++i) {

48             printf((i % 8 == 7 ? "%3d\n" : "%3d "), dst[i]);

49         }

50         printf("\n");

51     }

52     

53     return 0;

54 }