《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：

// 18.2 shuffle a deck of 52 cards, it must be perfect random.
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <vector>
using namespace std;

void printCards(const vector<int> &cards)
{
    int i;
    int n = (int)cards.size();
    const int col = 8;
    
    for (i = 0; i < n; ++i) {
        printf((i % col == col - 1 ? "%4d\n" : "%4d "), cards[i]);
    }
    printf("\n");
}

void shuffleCards(vector<int> &cards)
{
    vector<int> v;
    
    v = cards;
    int i, j;
    int n, n0;
    int idx;
    
    n0 = n = (int)cards.size();
    for (i = 0; i < n0; ++i) {
        idx = rand() % n;
        cards[i] = v[idx];
        --n;
        for (j = idx; j < n; ++j) {
            v[j] = v[j + 1];
        }
    }
    
    v.clear();
}

int main()
{
    srand((unsigned)time(NULL));
    vector<int> cards;
    int i;
    const int n = 52;
    
    cards.resize(n);
    for (i = 0; i < n; ++i) {
        cards[i] = i;
    }
    
    shuffleCards(cards);
    printCards(cards);
    
    return 0;
}