3577. Count the Number of Computer Unlocking Permutations

You are given an array complexity of length n.

There are n locked computers in a room with labels from 0 to n - 1, each with its own unique password. The password of the computer i has a complexity complexity[i].

The password for the computer labeled 0 is already decrypted and serves as the root. All other computers must be unlocked using it or another previously unlocked computer, following this information:

• You can decrypt the password for the computer i using the password for computer j, where j is any integer less than i with a lower complexity. (i.e. j < i and complexity[j] < complexity[i])
• To decrypt the password for computer i, you must have already unlocked a computer j such that j < i and complexity[j] < complexity[i].

Find the number of permutations of [0, 1, 2, ..., (n - 1)] that represent a valid order in which the computers can be unlocked, starting from computer 0 as the only initially unlocked one.

Since the answer may be large, return it modulo 109 + 7.

Note that the password for the computer with label 0 is decrypted, and not the computer with the first position in the permutation.

A permutation is a rearrangement of all the elements of an array.

class Solution {
public:
    int countPermutations(vector<int>& complexity) {
        long long mod = 1e9 + 7, res = 1;
        for(int i = 1; i < complexity.size(); i++) {
            if(complexity[i] <= complexity[0]) return 0;
            res = res * i % mod;
        }
        return res;
    }
};