#include <bits/stdc++.h>
using namespace std;
// * No One Dies a Virgin, Life Fucks Us All
typedef long long ll;
#define nl '\n'
#define ff first
#define ss second
#define pb push_back
#define sik(x) {cout << x << nl; return;}
constexpr ll maxn = 2005, mod = 1e9 + 7, inf = 1e17, SQ = 450, LG = 20;
typedef pair<int, int> pii;
int n, S, T;
ll dp[maxn][maxn][maxn], pd[maxn][maxn][maxn];
constexpr inline void md(ll& a) {
while (a >= mod) a -= mod;
while (a < 0) a += mod;
}
int32_t main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> S >> T;
dp[1][1][(S == 1 || T == 1)] = 1;
for (int i = 2 ; i <= n ; i ++) {
for (int j = 1 ; j < i ; j ++) {
if (i == S || i == T) {
for (int k : {0, 1}) {
md(dp[i][j + 1][k + 1] += dp[i - 1][j][k]);
md(dp[i][j][k + 1] += dp[i - 1][j][k]);
}
} else {
for (int k : {0, 1, 2}) {
dp[i][j + 1][k] = (dp[i][j + 1][k] + dp[i - 1][j][k] * (j + 1 - k)) % mod;
dp[i][j - 1][k] = (dp[i][j - 1][k] + dp[i - 1][j][k] * (j - 1)) % mod;
}
}
}
}
cout << dp[n][1][2] % mod << endl;
}
Compilation message (stderr)
/usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(ios_init.o): in function `std::ios_base::Init::Init()':
(.text._ZNSt8ios_base4InitC2Ev+0x1f): failed to convert GOTPCREL relocation against '_ZNSt8ios_base4Init11_S_refcountE'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x5f): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0x66): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0x71): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0x7c): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xa0): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xab): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xc8): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xe1): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xf4): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cin_sync' defined in .bss._ZN14__gnu_internal12buf_cin_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0xfb): relocation truncated to fit: R_X86_64_PC32 against symbol `__gnu_internal::buf_cout_sync' defined in .bss._ZN14__gnu_internal13buf_cout_syncE section in /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.a(globals_io.o)
(.text._ZNSt8ios_base4InitC2Ev+0x101): additional relocation overflows omitted from the output
(.text._ZNSt8ios_base4InitC2Ev+0x1ed): failed to convert GOTPCREL relocation against '_ZSt4cout'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x252): failed to convert GOTPCREL relocation against '_ZSt3cin'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x2bc): failed to convert GOTPCREL relocation against '_ZSt4cerr'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x316): failed to convert GOTPCREL relocation against '_ZSt4clog'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x50f): failed to convert GOTPCREL relocation against '_ZSt5wcout'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x57d): failed to convert GOTPCREL relocation against '_ZSt4wcin'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x5f0): failed to convert GOTPCREL relocation against '_ZSt5wcerr'; relink with --no-relax
(.text._ZNSt8ios_base4InitC2Ev+0x654): failed to convert GOTPCREL relocation against '_ZSt5wclog'; relink with --no-relax
/usr/bin/ld: final link failed
collect2: error: ld returned 1 exit status
