#include <bits/stdc++.h> using namespace std; #define ll long long #define pb push_back #define ins insert #define fi first #define se second #define MASK(x) (1ULL << (x)) #define BIT(x, k) ((x) >> (k) & 1) #define ALL(x) (x).begin(), (x).end() #define TIME (1.0 * clock() / CLOCKS_PER_SEC) #define file "KANGAROO" template<typename T1, typename T2> bool minimize(T1 &a, const T2 &b) {if (a > b) a = b; else return 0; return 1;} template<typename T1, typename T2> bool maximize(T1 &a, const T2 &b) {if (a < b) a = b; else return 0; return 1;} void fastio(){ if (fopen(file".INP", "r")){ freopen(file".INP", "r", stdin); freopen(file".OUT", "w", stdout); } } const int maxn = 2e3 + 5; const int mod = 1e9 + 7; void add(int &a, const int &b){ a += b; if (a >= mod) a -= mod; } int mul(const int &a, const int &b){ return (1ll * a * b) % mod; } int n, st, en; int dp[maxn][maxn]; signed main(){ ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0); fastio(); cin >> n >> st >> en; dp[0][0] = 1; for(int i = 1; i <= n; ++i) for(int j = 1; j <= i; ++j){ //i = st hoac i = en if (i == st || i == en){ add(dp[i][j], dp[i - 1][j]); add(dp[i][j], dp[i - 1][j - 1]); } //i khac st va en else{ //Noi 2 tplt if (j < i) add(dp[i][j], mul(dp[i - 1][j + 1], j)); //Tao 1 tplt moi add(dp[i][j], mul(dp[i - 1][j - 1], j - (i > st) - (i > en))); } } cout << dp[n][1] << '\n'; cerr << "\nTime ran : " << TIME << "s.\n"; return 0; }

kangaroo.cpp: In function 'void fastio()':
kangaroo.cpp:22:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   22 |                 freopen(file".INP", "r", stdin);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
kangaroo.cpp:23:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   23 |                 freopen(file".OUT", "w", stdout);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~