Codeforces 367E Sereja and Intervals (dp + sqrt)

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#include <iostream>  #include <cstdio>  #include <cstring>  #include <cmath>  #include <algorithm>  #include <queue>  #include <set>  #include <ctime>  #include <cstdlib>  using namespace std;  #define inf 0x3f3f3f3f#define N 100020#define LL long long#define mod 1000000007#define ls (i << 1)#define rs (ls | 1)#define md (ll + rr >> 1)#define lson ll, md, ls#define rson md + 1, rr, rs#define B 350int n, m, X;int dp[40000020];void add(int &x, int y) {x += y;if(x >= mod) x -= mod;}int &f(int i, int x, int y) {int t = i * (n + 2) * (n + 2) + x * (n + 2) + y;return dp[t];}int main() {scanf("%d%d%d", &n, &m, &X);if(n > m) {puts("0");return 0;}dp[0] = 1;for(int i = 1; i <= m; ++i) {for(int x = 0; x <= i && x <= n; ++x) {for(int y = 0; y <= x; ++y) {if(i == X) {if(y > 0) add(f(i, x, y), f(i - 1, x - 1, y - 1));else f(i, x, y) = 0;if(x > 0) add(f(i, x, y), f(i - 1, x - 1, y));}else {add(f(i, x, y), f(i - 1, x, y + 1));if(y > 0) add(f(i, x, y), f(i - 1, x - 1, y - 1));add(f(i, x, y), f(i - 1, x, y));if(x > 0) add(f(i, x, y), f(i - 1, x - 1, y));}}}}int ans = f(m, n, 0);for(int i = 1; i <= n; ++i) ans = 1LL * ans * i % mod;printf("%d\n", ans);return 0;}

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