CF370 D Memory and Scores

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dp题并运用了前缀和
我看题目提示中有fft 我想了下感觉复杂度不过关还是未解

#include<bits/stdc++.h>using namespace std;typedef long long ll;const int MAXN = 4e5+5;const int MOD = 1e9+7;const int ze = 2e5;ll dp[2][MAXN];ll sum[MAXN];int main(){    int a,b,k,t;    while(~scanf("%d %d %d %d",&a,&b,&k,&t)) {        memset(dp,0,sizeof(dp));        int fl= 1;        dp[1][ze] = 1; sum[0] = 0;        for(int i = 1; i <= t; ++i) {            for(int j = 1; j < MAXN; ++j) {                sum[j] = (dp[fl][j]+sum[j-1]) %MOD;                         }            memset(dp[fl^1],0,sizeof(dp[fl^1]));            for(int j = 1; j < MAXN; ++j) {                int t1 = max(0,j-k-1); int t2 = min(MAXN-1, j+k);                dp[fl^1][j] = (dp[fl^1][j]+sum[t2]-sum[t1]+MOD) %MOD;            }            fl ^= 1;        }        for(int i = 1; i < MAXN; ++i) {            sum[i] = (dp[fl][i] + sum[i-1]) %MOD;        }        ll ans = 0;        for(int i = 1; i < MAXN; ++i) {            int tt = a-b+i;            if(tt >= MAXN) tt = MAXN;            if(tt < 1) tt = 1;            ans = (ans + dp[fl][i]*sum[tt-1]%MOD)%MOD;        }        printf("%lld\n",ans);    }    return 0;}

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