http://codeforces.com/problemset/problem/476/B 题意：给出两个长度相等-且不超过10的字符串，串1只包含‘-’,’+‘。按照‘+’为1，‘-’为-1累加可以得到一个值。串2还包含若干‘？’，代表该处的值不确定，且为’+‘和’-‘的概率相等，都是0.5.问串2的值和串1相等的概率。 思路：我们可以扫一遍得到‘？’的个数和两个式子的差值。设问号个数为a，差值为b，那么在a个问号中需要有(a-b)/2个为‘+’（容易知道，a,b一定奇偶性相同，所以a-b一定能被2整除），根据超几何分布，概率为 c[a][(a-b)/2]*(1/2)^a; 写的时候可以先打个组合数的表。1A，开心。
http://codeforces.com/contest/621/problem/C
C. Wet Shark and Flowers
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output
There are n sharks who grow flowers for Wet Shark. They are all sitting around the table, such that sharks i and i + 1 are neighbours for all i from 1 to n - 1. Sharks n and 1 are neighbours too.
Each shark will grow some number of flowers s__i. For i-th shark value s__i is random integer equiprobably chosen in range from _l__i_to r__i. Wet Shark has it’s favourite prime number p, and he really likes it! If for any pair of neighbouring sharks i and j the product s__i·s__j is divisible by p, then Wet Shark becomes happy and gives 1000 dollars to each of these sharks.
http://codeforces.com/contest/30/problem/C 题意：给出n个target在一个二维平面上。给出每个target的坐标，出现的时间，以及击中的概率。target出现之后就会瞬间消失，枪移动的单位速度为1，射击不需要时间。问能击中的target的最大期望是多少。
Desiderium # **Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 65536/65536 K (Java/Others) Total Submission(s): 427 Accepted Submission(s): 167 **
Problem Description
There is a set of intervals, the size of this set is n.
If we select a subset of this set with equal probability, how many the expected length of intervals’ union of this subset is?
B. Andrey and Problem
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output
Andrey needs one more problem to conduct a programming contest. He has n friends who are always willing to help. He can ask some of them to come up with a contest problem. Andrey knows one value for each of his fiends – the probability that this friend will come up with a problem if Andrey asks him.
