PKU_campus_2018_D Chocolate

思路：

题目链接http://poj.openjudge.cn/practice/C18D/

kruskal过程中使用乘法原理计数。

实现：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = 405, INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
string s[MAXN];
int d[MAXN][MAXN];
int par[MAXN]; 
ll num[MAXN];
struct edge
{
    int a, b, l;
};
edge es[MAXN * MAXN];
void init(int n)
{
    for (int i = 0; i < n; i++) { par[i] = i; num[i] = 1; }
}
int find(int x)
{
    if (par[x] == x) return x;
    return par[x] = find(par[x]);
}
int uni(int x, int y)
{
    x = find(x); y = find(y);
    if (x == y) return x;
    par[x] = y; return y;
}
int edit_dis(string a, string b)
{
    int la = a.length(), lb = b.length();
    for (int i = 0; i <= la; i++) d[i][0] = i;
    for (int i = 0; i <= lb; i++) d[0][i] = i;
    for (int i = 1; i <= la; i++)
    {
        for (int j = 1; j <= lb; j++)
        {
            if (a[i - 1] == b[j - 1]) d[i][j] = d[i - 1][j - 1];
            else d[i][j] = min(d[i][j - 1], d[i - 1][j]) + 1;
        }
    }
    return d[la][lb];
}
bool cmp(edge & x, edge & y)
{
    return x.l < y.l;
}
bool check(int x, int cnt)
{
    int minn = INF, maxn = -INF;
    for (int i = 0; i < cnt; i++)
    {
        int px = find(es[i].a), py = find(es[i].b);
        if (px == x && py == x) maxn = max(maxn, es[i].l);
        else if ((px == x && py != x) || (px != x && py == x))
            minn = min(minn, es[i].l);
    }
    return maxn < minn;
}
int main()
{
    int t, n;
    cin >> t;
    while (t--)
    {
        cin >> n;
        init(n);
        for (int i = 0; i < n; i++) cin >> s[i];
        int cnt = 0;
        for (int i = 0; i < n; i++)
        {
            for (int j = i + 1; j < n; j++)
            {
                es[cnt].a = i; es[cnt].b = j; es[cnt].l = edit_dis(s[i], s[j]);
                cnt++;
            }
        }
        sort(es, es + cnt, cmp);
        ll ans = 0; int tot = n;
        for (int i = 0; i < cnt; i++)
        {
            int px = find(es[i].a), py = find(es[i].b);
            if (px == py) continue;
            int tmp = uni(es[i].a, es[i].b);
            num[tmp] = num[px] * num[py] % MOD;
            if (check(tmp, cnt)) num[tmp] = (num[tmp] + 1) % MOD;
            tot--;
            if (tot == 1) ans = num[tmp];
        }
        cout << ans << endl;
    }
    return 0;
}