Poj2795Exploring PyramidsDp

题意：给出一个字符串。问有多少个满足以下条件的树

从原点开始尽可能左走，不行就回溯，其路径符合给出字符串。

#include <cstdio>

#include <cstring>

#include <algorithm>

#include <climits>

#include <string>

#include <iostream>

#include <map>

#include <cstdlib>

#include <list>

#include <set>

#include <queue>

#include <stack>

#include <math.h>

using namespace std;



typedef long long LL;

const LL mod = 1000000000;

const LL maxn = 300 + 10;

LL dp[maxn][maxn];

char s[maxn];

LL gao(LL i, LL j)

{

    if (~dp[i][j]) return dp[i][j];

    if (i == j) return dp[i][j] = 1;

    if (s[i] != s[j]) return dp[i][j] = 0;

    LL ans = 0;

    for (LL x = i + 2; x <= j; x++){

        if (s[i] != s[x]) continue;

        ans += gao(i + 1, x - 1) * gao(x, j);

        ans %= mod;

    }

    return dp[i][j] = ans;

}



int main()

{

    while (cin >> s){

        LL len = strlen(s);

        memset(dp, -1, sizeof(dp));

        cout << gao(0, len - 1)<<endl;

    }

    return 0;

}