HDU 6148.Valley Numer

怀疑题目名称真的没错?

数位 DP 裸题,但是有很多细节要注意。
实在不行可以手动模拟。

代码:

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#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int LEN = 100;
const int mod = 1e9 + 7;
int T;
char d[LEN + 5];
int tot;
long long f[LEN + 5][10][2][2];
long long dfs(int x,int num,int up,int lead,int top)
{
if(!x)
return 1;
if(!top && ~f[x][num][up][lead])
return f[x][num][up][lead];
long long ret = 0;
int bound = top ? d[x] - '0' : 9;
for(register int i = x > 1 || !lead ? 0 : 1;i <= bound;++i)
{
if(up && num > i)
continue;
ret = (ret + dfs(x - 1,i,(num < i || (up && num == i)) && !lead,lead && !i,top && i == bound)) % mod;
}
if(!top)
f[x][num][up][lead] = ret;
return ret;
}
int main()
{
scanf("%d",&T);
while(T--)
{
memset(f,-1,sizeof f);
scanf("%s",d + 1);
tot = strlen(d + 1);
reverse(d + 1,d + tot + 1);
printf("%lld\n",dfs(tot,0,0,1,1));
}
}