UVA10917 Walk Through the Forest -- 迪界特斯拉

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本文链接： https://blog.csdn.net/lmhlmh_/article/details/102400054

先dij算出每个点到目的点的最短距离d，对于AB边，要求d[B]<d[A]即可，然后记忆化搜索

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;
typedef long long ll;
const int INF = 2e9;
const int maxn = 1010;
int n,m;
struct Edge {
    int to,dist;
} ;
vector<Edge> edges;
int inde;
vector<int> G[maxn];
int d[maxn];
bool used[maxn];
int dp[maxn];
struct Node {
    int u,d;
    bool operator < (const Node &n) const {
        return d > n.d;
    }
};

void add_edge(int u,int v,int d) {
    edges.push_back((Edge){v,d});
    G[u].push_back(inde++);
};

void Init() {
    edges.clear();
    inde = 0;
    for(int i = 0;i < n;i++) G[i].clear();
}

void dij(int u,int *d) {
    memset(used,0,sizeof(used));
    for(int i = 0;i < n;i++) d[i] = INF;
    d[u] = 0;
    priority_queue<Node> q;
    q.push((Node){u,0});
    while(!q.empty()) {
        Node cur = q.top();
        q.pop();
        int u = cur.u;
        if(used[u]) continue;
        used[u] = true;
        for(int i = 0;i < G[u].size();i++) {
            Edge e = edges[G[u][i]];
            if(d[u]+e.dist < d[e.to]) {
                d[e.to] = d[u]+e.dist;
                q.push((Node){e.to,d[e.to]});
            }
        }
    }
}

int DP(int u) {
    if(u == 1) return 1;
    if(dp[u] >= 0) return dp[u];
    int cnt = 0;
    for(int i = 0;i < G[u].size();i++) {
        int v = edges[G[u][i]].to;
        if(d[v] >= d[u]) continue;
        cnt += DP(v);
    }
    return dp[u] = cnt;
}

int main() {
    while(scanf("%d",&n) == 1 && n) {
        scanf("%d",&m);
        Init();
        for(int i = 0;i < m;i++) {
            int a,b,c;
            scanf("%d%d%d",&a,&b,&c);
            a--;b--;
            add_edge(a,b,c);
            add_edge(b,a,c);
        }
        dij(1,d);
        for(int i = 0;i < n;i++) dp[i] = -1;
        printf("%d\n",DP(0));
    }

    return 0;
}