# 1997. First Day Where You Have Been in All the Rooms
###### tags: `Leetcode` `Medium` `Dynamic Programming`
Link: https://leetcode.com/problems/first-day-where-you-have-been-in-all-the-rooms/description/
## 思路

第```i```个room只能从第```i-1```个room到达 第一次到达第```i-1```个room的时间是```dp[i-1]``` 到达之后将要去```nextVisit[i-1]``` 那么从```nextVisit[i-1]```再到```i-1```需要```dp[i-1]-dp[nextVisit[i-1]]```天 所以```dp[i] = 2*dp[i-1]-dp[nextVisit[i-1]]+2```
## Code

class Solution:
def firstDayBeenInAllRooms(self, nextVisit: List[int]) -> int:
n, mod = len(nextVisit), 1e9+7
dp = [0]*n
for i in range(1, n):
dp[i] = int((2*dp[i-1]-dp[nextVisit[i-1]]+2)%mod)
return dp[-1]