题意：http://www.cnblogs.com/zhyfzy/p/4436017.html
题解：概率DP，算第几次拿到i的概率 ，把总和加起来再乘以 d[i]就行了
解题代码：
1 // BEGIN CUT HERE 2 /* 3 4 */ 5 // END CUT HERE 6 #line 7 "RandomPancakeStack.cpp" 7 #include <cstdlib> 8 #include <cctype> 9 #include <cstring> 10 #include <cstdio> 11 #include <cmath> 12 #include <algorithm> 13 #include <vector> 14 #include <string> 15 #include <iostream> 16 #include <sstream> 17 #include <map> 18 #include <set> 19 #include <queue> 20 #include <stack> 21 #include <fstream> 22 #include <numeric> 23 #include <iomanip> 24 #include <bitset> 25 #include <list> 26 #include <stdexcept> 27 #include <functional> 28 #include <utility> 29 #include <ctime> 30 using namespace std; 31 32 #define PB push_back 33 #define MP make_pair 34 35 #define REP(i,n) for(i=0;i<(n);++i) 36 #define FOR(i,l,h) for(i=(l);i<=(h);++i) 37 #define FORD(i,h,l) for(i=(h);i>=(l);--i) 38 39 typedef vector<int> VI; 40 typedef vector<string> VS; 41 typedef vector<double> VD; 42 typedef long long LL; 43 typedef pair<int,int> PII; 44 45 46 double dp[260][260]; 47 double add[260]; 48 class RandomPancakeStack 49 { 50 public: 51 double expectedDeliciousness(vector <int> d) 52 { 53 memset(dp,0,sizeof(dp)); 54 int n = d.size(); 55 for(int i = 0 ;i < n;i ++) 56 dp[0][i] = 1.0/n ; 57 58 for(int i = 1 ;i < n ;i ++){ 59 for(int j = 0 ;j < n;j ++){ 60 dp[i][j] = 0 ; 61 for(int k = j + 1; k < n;k++){ 62 dp[i][j] += dp[i-1][k] * 1./(n-i); 63 } 64 } 65 } 66 double ans = 0 ; ; 67 for(int i = 0 ;i < n;i ++) 68 { 69 double tmp =0 ; 70 for(int j = 0;j <= n;j ++) 71 tmp += dp[j][i]; 72 ans += tmp * d[i]; 73 } 74 return ans; 75 } 76 77 // BEGIN CUT HERE 78 public: 79 void run_test(int Case) { if ((Case == -1) || (Case == 0)) test_case_0(); if ((Case == -1) || (Case == 1)) test_case_1(); if ((Case == -1) || (Case == 2)) test_case_2(); if ((Case == -1) || (Case == 3)) test_case_3(); if ((Case == -1) || (Case == 4)) test_case_4(); if ((Case == -1) || (Case == 5)) test_case_5(); } 80 private: 81 template <typename T> string print_array(const vector<T> &V) { ostringstream os; os << "{ "; for (typename vector<T>::const_iterator iter = V.begin(); iter != V.end(); ++iter) os << '"' << *iter << "","; os << " }"; return os.str(); } 82 void verify_case(int Case, const double &Expected, const double &Received) { cerr << "Test Case #" << Case << "..."; if (Expected == Received) cerr << "PASSED" << endl; else { cerr << "FAILED" << endl; cerr << " Expected: "" << Expected << '"' << endl; cerr << " Received: "" << Received << '"' << endl; } } 83 void test_case_0() { int Arr0[] = {1,1,1}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 1.6666666666666667; verify_case(0, Arg1, expectedDeliciousness(Arg0)); } 84 void test_case_1() { int Arr0[] = {3,6,10,9,2}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 9.891666666666667; verify_case(1, Arg1, expectedDeliciousness(Arg0)); } 85 void test_case_2() { int Arr0[] = {10,9,8,7,6,5,4,3,2,1}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 10.999999724426809; verify_case(2, Arg1, expectedDeliciousness(Arg0)); } 86 void test_case_3() { int Arr0[] = {1,2,3,4,5,6,7,8,9,10}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 7.901100088183421; verify_case(3, Arg1, expectedDeliciousness(Arg0)); } 87 void test_case_4() { int Arr0[] = {2,7,1,8,2,8,1,8,2,8,4,5,90,4,5,2,3,5,60,2,8,74,7,1}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 19.368705050402465; verify_case(4, Arg1, expectedDeliciousness(Arg0)); } 88 void test_case_5() { int Arr0[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 89 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); double Arg1 = 1.718281828459045; verify_case(5, Arg1, expectedDeliciousness(Arg0)); } 90 91 // END CUT HERE 92 93 }; 94 95 // BEGIN CUT HERE 96 int main() 97 { 98 RandomPancakeStack ___test; 99 ___test.run_test(-1); 100 return 0; 101 } 102 // END CUT HERE