1857. Largest Color Value in a Directed Graph

class Solution:
    def largestPathValue(self, colors: str, edges: List[List[int]]) -> int:
        n = len(colors)
        indeg = [0] * n
        g = defaultdict(list)
        for a, b in edges:
            g[a].append(b)
            indeg[b] += 1
        q = deque()
        dp = [[0] * 26 for _ in range(n)]
        for i, v in enumerate(indeg):
            if v == 0:
                q.append(i)
                c = ord(colors[i]) - ord('a')
                dp[i][c] += 1
        cnt = 0
        res = 1
        while q:
            i = q.popleft()
            cnt += 1
            for j in g[i]:
                indeg[j] -= 1
                if indeg[j] == 0:
                    q.append(j)
                c = ord(colors[j]) - ord('a')
                for k in range(26):
                    dp[j][k] = max(dp[j][k], dp[i][k] + (c == k))
                    res = max(res, dp[j][k])
        return -1 if cnt < n else res