Codeforces Round #302 (Div. 二) D. Destroying Roads(最短路)

Codeforces Round #302 (Div. 2) D. Destroying Roads(最短路)

In some country there are exactly n cities and m bidirectional roads connecting the cities. Cities are numbered with integers from 1 to n. If cities a and b are connected by a road, then in an hour you can go along this road either from city a to city b, or from city b to city a. The road network is such that from any city you can get to any other one by moving along the roads.

You want to destroy the largest possible number of roads in the country so that the remaining roads would allow you to get from city s1 to city t1 in at most l1 hours and get from city s2 to city t2 in at most l2 hours.

Determine what maximum number of roads you need to destroy in order to meet the condition of your plan. If it is impossible to reach the desired result, print -1.

Input

The first line contains two integers n, m (1 ≤ n ≤ 3000, $Codeforces Round #302 (Div. 二) D. Destroying Roads(最短路)$ ) — the number of cities and roads in the country, respectively.

Next m lines contain the descriptions of the roads as pairs of integers ai, bi (1 ≤ ai, bi ≤ n, ai ≠ bi). It is guaranteed that the roads that are given in the description can transport you from any city to any other one. It is guaranteed that each pair of cities has at most one road between them.

The last two lines contains three integers each, s1, t1, l1 and s2, t2, l2, respectively (1 ≤ si, ti ≤ n, 0 ≤ li ≤ n).

Output

Print a single number — the answer to the problem. If the it is impossible to meet the conditions, print -1.

Sample test(s)

input
5 4
1 2
2 3
3 4
4 5
1 3 2
3 5 2

output
0

input
5 4
1 2
2 3
3 4
4 5
1 3 2
2 4 2

output
1

input
5 4
1 2
2 3
3 4
4 5
1 3 2
3 5 1

output
-1

n个结点,m条边的图(边权全为1),问最多能删掉多少条边使得s1到t1距离不超过l1,s2到t2距离不超过l2。

思路:其实题目的意思也就是说最少需要多少条边使得s1到t1距离不超过l1且s2到t2距离不超过l2.如果需要的边没有相交那么显然是两个最短路上边,如果相交那么应该是相交部分的最短路和相交的两个端点到其他四个点的最短路，所以我们可以先预处理出任意两点间的最短路,然后枚举相交部分的两个端点,最后用总边数减去需要的边数就是答案

#include <bits/stdc++.h>
#define foreach(it,v) for(__typeof((v).begin()) it = (v).begin(); it != (v).end(); ++it)
using namespace std;
typedef long long ll;
const int inf = 1e7;
const int maxn = 3000 + 5;
int n,m;
int d[maxn][maxn];
vector<int> G[maxn];
void AddEdge(int u,int v)
{
    G[u].push_back(v);
    G[v].push_back(u);
}
void bfs(int cur)
{
    memset(d[cur],0,sizeof d[cur]);
    queue<int>q;
    q.push(cur);
    while(!q.empty()) {
        int u = q.front(); q.pop();
        foreach(it,G[u])if(!d[cur][*it]){
            d[cur][*it] = d[cur][u] + 1;
            q.push(*it);
        }
    }
    for(int i = 1; i <= n; i++) if(!d[cur][i])
        d[cur][i] = inf;
    d[cur][cur] = 0;
}
int main()
{
    while(~scanf("%d%d",&n,&m)) {
       for(int i = 1; i <= n; i++)G[i].clear();
        for(int i = 1; i <= m; i++) {
            int u,v;scanf("%d%d",&u,&v);
            AddEdge(u,v);
        }
        for(int i = 1; i <= n; i++)bfs(i);
        int s1,t1,l1,s2,t2,l2;
        scanf("%d%d%d%d%d%d",&s1,&t1,&l1,&s2,&t2,&l2);

        if(d[s1][t1]>l1||d[s2][t2]>l2) {
            puts("-1");
            continue;
        }
        int res = d[s1][t1]+d[s2][t2];
        for(int i = 1; i <= n; i++) {
            for(int j = i; j <= n; j++) {
                int w1 = min(d[s1][i]+d[i][j]+d[j][t1], d[s1][j]+d[j][i]+d[i][t1]);
                int w2 = min(d[s2][i]+d[i][j]+d[j][t2], d[s2][j]+d[j][i]+d[i][t2]);
                if(w1>l1||w2>l2)continue;
                res = min(res,w1+w2-d[i][j]);
            }
        }
        printf("%d\n", m - res);
    }
    return 0;
}

Codeforces Round #302 (Div. 二) D. Destroying Roads(最短路)

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