hdu1142 A Walk Through the Forest

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题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1142

题意：问从office到home有多少路可以走，下一步到家的最小距离必须比现在到家的最短距离小才能到下一步。

思路：先处理出各个点到home的最短距离，再用dfs+记忆化搜索

#include<iostream>#include<cstdio>#include<cstring>#include<algorithm>#include<map>#include<queue>#define maxn 1005#define inf 100000using namespace std;int n, m, ans;int Map[maxn][maxn];bool vis[maxn];int dp[maxn];int dist[maxn];void pre(){    memset(vis, 0, sizeof(vis));    for(int i = 1; i <= n; i++)    {        for(int j = 1; j <= n; j++)        {            Map[i][j] = inf;        }    }}void Dijkstra(int cur = 2){    vis[cur] = 1;    for(int i = 1; i <= n; i++)        dist[i] = Map[cur][i];    dist[cur] = 0;    for(int i = 2; i <= n; i++)    {        int k = cur;        int minval = inf;        for(int j = 1; j <= n; j++)        {            if(!vis[j] && dist[j] < minval)            {                minval = dist[j];                k = j;            }        }        vis[k] = 1;        for(int j = 1; j <= n; j++)        {            if(!vis[j])                dist[j] = min(dist[j], dist[k] + Map[k][j]);        }    }}int DFS(int node){    if(dp[node])    {        return dp[node];    }    int ans = 0;    for(int i = 1; i <= n; i++)    {        if(Map[node][i] != inf && dist[i] < dist[node])        {            ans += DFS(i);        }    }    return dp[node] = ans;}int main(){    while(~scanf("%d", &n) && n)    {        scanf("%d", &m);        pre();        for(int i = 0; i < m; i++)        {            int s, e, dis;            scanf("%d %d %d", &s, &e, &dis);            Map[s][e] = Map[e][s] = dis;        }        Dijkstra();        dp[2] = vis[1] = 1;        ans = DFS(1);        cout << ans << endl;    }    return 0;}

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