ch_5302 金字塔

区间dp，递归写法，

#include<iostream>
#include<cstdio>

#define ri register int
#define u long long

namespace opt {

    inline u in() {
        u x(0),f(1);
        char s(getchar());
        while(s<'0'||s>'9') {
            if(s=='-') f=-1;
            s=getchar();
        }
        while(s>='0'&&s<='9') {
            x=(x<<1)+(x<<3)+s-'0';
            s=getchar();
        }
        return x*f;
    }

}

using opt::in;

#define NN 305
#define MO 1000000000

#include<algorithm>
#include<cstring>

namespace mainstay {

    char s[NN];

    u f[NN][NN];

    u dfs(const u &l,const u &r) {
        if(s[l]!=s[r]) return 0;
        if(f[l][r]!=-1) return f[l][r];
        if(l==r) return 1;
        if(l>r) return 0;
        f[l][r]=0;
        for(ri i(l+2); i<=r; ++i) f[l][r]=(f[l][r]+dfs(l+1,i-1)*dfs(i,r)%MO)%MO;
        return f[l][r];
    }

    inline void solve() {
        scanf("%s",s);
        memset(f,-1,sizeof(f));
        u len(std::strlen(s));
        std::cout<<dfs(0,len-1);
    }

}

int main() {

    //freopen("x.txt","r",stdin);
    //freopen("my.txt","w",stdout);
    std::ios::sync_with_stdio(false);
    mainstay::solve();

}