[leetcode]Maximum Vacation Days

思路简单，状态转移方程容易想。需要注意的是转移时的一些状态，比如是否连通，等等。

class Solution:
    def maxVacationDays(self, flights: List[List[int]], days: List[List[int]]) -> int:
        
        n = len(flights)
        k = len(days[0])
        
        adjLists = {} 
        
        for i in range(n):
            for j in range(n):
                if flights[i][j] == 1 or i == j: # i --> j
                    if j not in adjLists:
                        adjLists[j] = []
                    adjLists[j].append(i)
                    
        
        memo = {} # i, j : i - city; j - week;  max days for vacation
        
        result = 0
        
        for j in range(k):
            for i in range(n):
                if j == 0: # first week
                    if 0 in adjLists[i]:
                        memo[(i, j)] = days[i][j]
                    else:
                        memo[(i, j)] = -1
                else: # j > 0
                    maxTmp = -1
                    for city in adjLists[i]:
                        maxTmp = max(memo[(city, j - 1)], maxTmp)
                    if maxTmp != -1:
                        memo[(i, j)] = maxTmp + days[i][j]
                    else:
                        memo[(i, j)] = -1
                    
                result = max(result, memo[(i, j)])
        
        return result