ABC189F题

题目链接: https://atcoder.jp/contests/abc189/tasks/abc189_f.
难点在于有返回0广场时，旋转轮子次数的期望值。

代码如下：

#include<iostream>
#include<cstdio>
using namespace std;
const int BUF = 100005;
 
 
int nCell, nMove, nBackCell;
int backCell[BUF];
 
void read() {
    
    
    cin >> nCell >> nMove >> nBackCell;
    for (int i = 0; i < nBackCell; ++i) {
    
    
        cin >> backCell[i];
    }
}
 
 
void work() {
    
    
    bool isBackCell[BUF] = {
    
    };
    for (int i = 0; i < nBackCell; ++i) {
    
    
        isBackCell[backCell[i]] = true;
    }
 
    double goBackP[BUF * 2], curGoBack = 0;
    double ans[BUF * 2], curAns = 0;
    for (int i = nCell; i < nCell + nMove; ++i) {
    
    
        goBackP[i] = ans[i] = 0;
    }
    
    for (int i = nCell - 1; i >= 0; --i) {
    
    
        if (isBackCell[i]) {
    
    
            goBackP[i] = 1.0;
            ans[i] = 0;
        } else {
    
    
            goBackP[i] = curGoBack;
            ans[i] = curAns + 1;
        }
 
        curGoBack += (goBackP[i] - goBackP[i + nMove]) / nMove;
        curAns += (ans[i] - ans[i + nMove]) / nMove;
        
        if (curGoBack >= 1 - 1e-8) {
    
    
            cout << -1 << endl;
            return;
        }
    }
 
    printf("%.10lf\n", ans[0] / (1 - goBackP[0]));
}
 
 
int main() {
    
    
    read();
    work();
    return 0;
}