Code : Lining Up

Submission #include <bits/stdc++.h> using namespace std; const int mod = (int)1e9 + 7; void mult(int &a, int b) { long long res = 1LL * a * b; a = res % mod; } int solve(vector<int> &a) { int n = a.size(); map<int, int> slots; for (int i = 0; i < n; i++) { int diff = abs(i - (n - i - 1)); slots[diff]++; } int ans = 1; for (int i = 0; i < n; i++) { if (slots[a[i]] == 0) { return 0; } mult(ans, slots[a[i]]); slots[a[i]]--; } for (auto &kv : slots) { if (kv.