Ural_1036. Lucky Tickets(DP + 高精度)

　　纠结的高精度，终于拿下了。。。转移方程很简单 dp[i][j] = dp[i-1][j-k] {k| 0, 1, 2 .... , 9}，高精度整了一下午。悲剧，各种wa。。。重新敲了一遍，终于过了。

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

const int N = 550;
const int M = 50;
const int inf = 10000;

int dp[M+5][N][M+5];

void add(int a[M+1], int b[M+1]){
    int i;
    for(i = 1; i <= M; i++){
        a[i] += b[i];
    }
    for(i = 1; i <= M; i++){
        a[i+1] += a[i]/inf;
        a[i] %= inf;
    }
}

void mul(int a[M+1], int b[M+1]){
    int c[2*M+1], i, j;
    memset(c, 0, sizeof(c));
    for(i = 1; i <= M; i++)
        for(j = 1; j <= M; j++)
            c[i+j-1] += a[i]*b[j];
    for(i = 1; i <= M; i++){
        c[i+1] += c[i]/inf;
        c[i] %= inf;
    }
    for(i = 1; i <= M; i++)
        a[i] = c[i];
}

void display(int a[M+1]){
    int j, i = M;
    while(a[i] == 0 && i > 0)    i--;
    if(i == 0)    printf("0\n");
    else{
        printf("%d", a[i]);
        for(j = i-1; j > 0; j--){
            printf("%04d", a[j]);
        }
    }
    cout << endl;
}
int main(){
    //freopen("data.in", "r", stdin);

    int n, s, i, j, k;
    while(~scanf("%d%d", &n, &s)){
        if(s&1){
            printf("0\n");
            continue;
        } else {
            memset(dp, 0, sizeof(dp));

            for(i = 0; i < 10; i++)
                dp[1][i][1] = 1;

            for(i = 1; i <= n; i++)
                for(j = 0; j <= s/2; j++)
                    for(k = 0; k <= 9 && j >= k; k++)
                        add(dp[i][j], dp[i-1][j-k]);
            mul(dp[n][s/2], dp[n][s/2]);
            display(dp[n][s/2]);
        }
    }
    return 0;
}