HDU4405（期望dp）

标准期望套路，很水。读题看好是到n就可以停止了。

 const int maxn = 1e5 + ;
 int n, m;
 db dp[maxn];
 map<int, int> mp;

 int main() {
     while (~scanf("%d %d", &n, &m) && n) {
         rep(i, n, n + )    dp[i] = ;
         mp.clear();
         rep(i, , m) {
             int a, b;
             read(a), read(b);
             mp[a] = b;
         }

         db sum = ;
         irep(i, n - , ) {
             if (mp.count(i)) {
                 dp[i] = dp[mp[i]];
             } else {
                 dp[i] = sum /  + 1.0;
             }
             sum += dp[i] - dp[i + ];
         }

         printf("%.4lf\n", dp[]);
     }
     return ;
 }