《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 = ;

     for (i = ; i < n; ++i) {
         printf((i % col == col -  ? "%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 = ; i < n0; ++i) {
         idx = rand() % n;
         cards[i] = v[idx];
         --n;
         for (j = idx; j < n; ++j) {
             v[j] = v[j + ];
         }
     }

     v.clear();
 }

 int main()
 {
     srand((unsigned)time(NULL));
     vector<int> cards;
     int i;
     const int n = ;

     cards.resize(n);
     for (i = ; i < n; ++i) {
         cards[i] = i;
     }

     shuffleCards(cards);
     printCards(cards);

     return ;
 }