LeetCode-in-Java

1916. Count Ways to Build Rooms in an Ant Colony

Hard

You are an ant tasked with adding n new rooms numbered 0 to n-1 to your colony. You are given the expansion plan as a 0-indexed integer array of length n, prevRoom, where prevRoom[i] indicates that you must build room prevRoom[i] before building room i, and these two rooms must be connected directly. Room 0 is already built, so prevRoom[0] = -1. The expansion plan is given such that once all the rooms are built, every room will be reachable from room 0.

You can only build one room at a time, and you can travel freely between rooms you have already built only if they are connected. You can choose to build any room as long as its previous room is already built.

Return the number of different orders you can build all the rooms in. Since the answer may be large, return it modulo 109 + 7.

Example 1:

Input: prevRoom = [-1,0,1]

Output: 1

Explanation: There is only one way to build the additional rooms: 0 → 1 → 2

Example 2:

Input: prevRoom = [-1,0,0,1,2]

Output: 6

Explanation: The 6 ways are:

0 → 1 → 3 → 2 → 4

0 → 2 → 4 → 1 → 3

0 → 1 → 2 → 3 → 4

0 → 1 → 2 → 4 → 3

0 → 2 → 1 → 3 → 4

0 → 2 → 1 → 4 → 3

Constraints:

• n == prevRoom.length
• 2 <= n <= 105
• prevRoom[0] == -1
• 0 <= prevRoom[i] < n for all 1 <= i < n
• Every room is reachable from room 0 once all the rooms are built.

Solution

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

@SuppressWarnings("unchecked")
public class Solution {
    private static final int MOD = 1000000007;
    private List<Integer>[] graph;
    private long[] fact;

    public int waysToBuildRooms(int[] prevRoom) {
        int n = prevRoom.length;
        graph = new ArrayList[n];
        Arrays.setAll(graph, e -> new ArrayList<>());
        fact = new long[prevRoom.length + 10];
        fact[0] = fact[1] = 1;
        for (int i = 2; i < fact.length; i++) {
            fact[i] = fact[i - 1] * i;
            fact[i] %= MOD;
        }
        for (int i = 1; i < prevRoom.length; i++) {
            int pre = prevRoom[i];
            graph[pre].add(i);
        }

        long[] res = dfs(0);
        return (int) (res[1] % MOD);
    }

    private long[] dfs(int root) {
        long[] res = new long[] {1, 0};
        int cnt = 0;
        List<long[]> list = new ArrayList<>();
        for (int next : graph[root]) {
            long[] v = dfs(next);
            cnt += (int) v[0];
            list.add(v);
        }
        res[0] += cnt;
        long com = 1;
        for (long[] p : list) {
            long choose = c(cnt, (int) (p[0]));
            cnt -= p[0];
            com = com * choose;
            com %= MOD;
            com = com * p[1];
            com %= MOD;
        }
        res[1] = com;
        return res;
    }

    private long c(int i, int j) {
        long mod = 1000000007;
        long prevRoom = fact[i];
        long b = ((fact[i - j] % mod) * (fact[j] % mod)) % mod;
        BigInteger value = BigInteger.valueOf(b);
        long binverse = value.modInverse(BigInteger.valueOf(mod)).longValue();
        return (prevRoom * (binverse % mod)) % mod;
    }
}

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