poj 3255（次短路）

题意：给一张无向带权图，让你求总1-n的次短路。

思路：好久没写dij了这次敲一边果然敲错了。。。这题就是在dis上加一维dis[i][0]记录最短路dis[i][1]记录次短路。就和我们在一个数组中找最小值次小值的原理差不多。我们只需先更新最小值，更新好了不要忘了再用原来的最小值去更新次小值（我就忘了这个wa了2次）。然后在更新次小值。

代码如下：

/**************************************************
 * Author     : xiaohao Z
 * Blog     : http://www.cnblogs.com/shu-xiaohao/
 * Last modified : 2014-01-27 15:21
 * Filename     : poj_3255.cpp
 * Description     : 
 * ************************************************/

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <map>
#define MP(a, b) make_pair(a, b)
#define PB(a) push_back(a)

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<unsigned int,unsigned int> puu;
typedef pair<int, double> pid;
typedef pair<ll, int> pli;
typedef pair<int, ll> pil;

const int INF = 0x3f3f3f3f;
const double eps = 1E-6;
const int LEN = 50010;
int n, m, dis[LEN][2];
vector<pii> Map[LEN];
struct S{
    int d, v, f;
};
S MS(int d, int v, int f){S ret;ret.d = d;ret.v = v;ret.f = f;return ret;}
struct cmp{
    bool operator() (S a, S b){return a.d > b.d;}
};

void Dijkstra(int vex){
    priority_queue<S, vector<S>, cmp> q;
    int vis[LEN][2] = {0};
    memset(dis, 0x3f, sizeof dis);
    dis[vex][0] = 0;
    q.push(MS(dis[vex][0], vex, 0));
    while(!q.empty()){
        S nvex = q.top(); q.pop();
        int nv = nvex.v, nf = nvex.f;
        if(vis[nv][nf])continue;
        vis[nv][nf] = 1;
        for(int i=0; i<Map[nv].size(); i++){
            int x = Map[nv][i].first, y = Map[nv][i].second;
            if(dis[x][0] > dis[nv][nf] + y){
                dis[x][1] = min(dis[x][1], dis[x][0]);
                dis[x][0] = dis[nv][nf] + y;
                q.push(MS(dis[x][0], x, 0));
            }
            else if(dis[nv][nf] + y != dis[x][0] && dis[x][1] > dis[nv][nf] + y){
                dis[x][1] = dis[nv][nf] + y;
                q.push(MS(dis[x][1], x, 1));
            }
        }
    }
}

int main()
{
    //freopen("in.txt", "r", stdin);
    int a, b, val;
    while(scanf("%d%d", &n, &m)!=EOF){
        for(int i=0; i<LEN; i++)Map[i].clear();
        for(int i=0; i<m; i++){
            scanf("%d%d%d", &a, &b, &val);
            Map[a].PB(MP(b, val));
            Map[b].PB(MP(a, val));
        }
        Dijkstra(1);
        printf("%d
", dis[n][1]);
    }
    return 0;
}