《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：

 // 18.3 pick m integers randomly from an array of n integer.
 #include <cstdio>
 #include <cstdlib>
 #include <ctime>
 #include <vector>
 using namespace std;

 void randomSubset(const vector<int> &src, vector<int> &dst, int m)
 {
     vector<int> v;
     int i, j;
     int idx;
     int n;

     v = src;
     n = (int)v.size();
     dst.resize(m);
     for (i = ; i < m; ++i) {
         idx = rand() % n;
         dst[i] = v[idx];
         --n;
         for (j = idx; j < n; ++j) {
             v[j] = v[j + ];
         }
     }
     v.clear();
 }

 int main()
 {
     int i;
     int n, m;
     vector<int> src;
     vector<int> dst;
     int cc;

     cc = ;
     while (scanf("%d%d", &n, &m) == ) {
         ++cc;
         srand((unsigned)(time(NULL) * cc));
         src.resize(n);
         for (i = ; i < n; ++i) {
             src[i] = i;
         }
         randomSubset(src, dst, m);
         for (i = ; i < m; ++i) {
             printf((i %  ==  ? "%3d\n" : "%3d "), dst[i]);
         }
         printf("\n");
     }

     return ;
 }