305239: CF995F. Cowmpany Cowmpensation
Memory Limit:256 MB
Time Limit:2 S
Judge Style:Text Compare
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Description
Cowmpany Cowmpensation
题意翻译
树形结构,给每个节点分配工资($[1, d]$),子节点不能超过父亲节点的工资,问有多少种分配方案。题目描述
Allen, having graduated from the MOO Institute of Techcowlogy (MIT), has started a startup! Allen is the president of his startup. He also hires $ n-1 $ other employees, each of which is assigned a direct superior. If $ u $ is a superior of $ v $ and $ v $ is a superior of $ w $ then also $ u $ is a superior of $ w $ . Additionally, there are no $ u $ and $ v $ such that $ u $ is the superior of $ v $ and $ v $ is the superior of $ u $ . Allen himself has no superior. Allen is employee number $ 1 $ , and the others are employee numbers $ 2 $ through $ n $ . Finally, Allen must assign salaries to each employee in the company including himself. Due to budget constraints, each employee's salary is an integer between $ 1 $ and $ D $ . Additionally, no employee can make strictly more than his superior. Help Allen find the number of ways to assign salaries. As this number may be large, output it modulo $ 10^9 + 7 $ .输入输出格式
输入格式
The first line of the input contains two integers $ n $ and $ D $ ( $ 1 \le n \le 3000 $ , $ 1 \le D \le 10^9 $ ). The remaining $ n-1 $ lines each contain a single positive integer, where the $ i $ -th line contains the integer $ p_i $ ( $ 1 \le p_i \le i $ ). $ p_i $ denotes the direct superior of employee $ i+1 $ .
输出格式
Output a single integer: the number of ways to assign salaries modulo $ 10^9 + 7 $ .
输入输出样例
输入样例 #1
3 2
1
1
输出样例 #1
5
输入样例 #2
3 3
1
2
输出样例 #2
10
输入样例 #3
2 5
1
输出样例 #3
15