《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 = 0; i < m; ++i) {
        idx = rand() % n;
        dst[i] = v[idx];
        --n;
        for (j = idx; j < n; ++j) {
            v[j] = v[j + 1];
        }
    }
    v.clear();
}

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

    cc = 0;
    while (scanf("%d%d", &n, &m) == 2) {
        ++cc;
        srand((unsigned)(time(NULL) * cc));
        src.resize(n);
        for (i = 0; i < n; ++i) {
            src[i] = i;
        }
        randomSubset(src, dst, m);
        for (i = 0; i < m; ++i) {
            printf((i % 8 == 7 ? "%3d\n" : "%3d "), dst[i]);
        }
        printf("\n");
    }
    
    return 0;
}