学步园

一切尽在代码中。。。。。。。。

/*
    dijkstra + heap，时间复杂度: O((n + e)log(n)).
    对于稠密图来说，仍然是dij+heap快，而且越稠密越快!
    用前向星来存图，空间复杂度为: O(m).
    更新时间: 2011.09.22
*/
#include <iostream>
#include <cstring>
#include <cmath>
#include <climits>
#include <algorithm>
#include <cstdio>
using namespace std;

typedef long long int64;
#define mem(a, b) memset(a, b, sizeof(a))
#define Sqr(x) ((x) * (x))
template <class T>
inline T Max(T a, T b) { if (a < b) a = b; return a; }
template <class T>
inline T Min(T a, T b) { if (a > b) a = b; return a; }
template <class T>
inline void Swap(T & a, T & b) { T dt = a; a = b; b = dt; }

const int maxn = 50005;
const int maxm = 50005 << 1; // 无向图.
const double EPS = 1e-10;
const int64 INF = (1LL << 60) + (1LL << 61);

struct EDGE
{
    int a, b, c;
    int next;
};

int n, m, ds, dt; // n个点，m条边，s为原点，t为终点。
EDGE edge[maxm]; // 前向星.
int edge_num;
int first[maxn]; // 记录该点的第一条边在edge[]的下标.
bool visited[maxn]; // 判断该点的最短路是否已经求出来了.
int64 ans;

struct Heap
{ // 如果速度慢，就不要用struct，直接作为一般参数和函数.
    int heapsize;
    int heap_v[maxn]; // heap_v[i]为堆中第i个位置所存的图中的结点.
int heap_map[maxn]; // heap_map[i]表示图中的点i在堆中的位置.
int64 heap_w[maxn]; // heap_w[i]为堆中第i个位置所存的为dist[heap_v[i]].
    void heap_swap(int i, int j)
    {
        Swap(heap_w[i], heap_w[j]);
        heap_map[heap_v[i]] = j;
        heap_map[heap_v[j]] = i;
        Swap(heap_v[i], heap_v[j]);
    }
    void heap_up(int i)
    {
        while (i != 1)
        {
            if (heap_w[i] < heap_w[i >> 1])
            {
                heap_swap(i, i >> 1);
                i >>= 1;
            }
            else break;
        }
    }
    void heap_down(int i)
    { // 注: 在dijkstra+heap没用到该函数.
        while ((i << 1) <= heapsize)
        {
            i <<= 1;
            if (i + 1 <= heapsize && heap_w[i + 1] < heap_w[i]) i++;
            if (heap_w[i] < heap_w[i >> 1])
            {
                heap_swap(i, i >> 1);
            }
            else break;
        }
    }
    void Delmin(void)
    {
        heap_swap(1, heapsize);
        heapsize--;
        heap_down(1);
    }
};

Heap hp;

void Init(void)
{
    int i, j, k;
    ans = 0; edge_num = 0;
    mem(visited, 0);
    fill(first, first + maxn, -1);
    visited[ds] = 1;

    for (i = 1; i <= n; i++)
    {
        hp.heap_v[i] = i;
        hp.heap_map[i] = i;
        hp.heap_w[i] = INF;
    }
    hp.heap_w[ds] = 0; hp.heapsize = n;
    hp.heap_up(hp.heap_map[ds]);
}

void AddEdge(int a, int b, int c)
{
    edge[edge_num].a = a, edge[edge_num].b = b, edge[edge_num].c = c;
    edge[edge_num].next = first[a], first[a] = edge_num++;
}

void Dijkstra_heap(void)
{ // hp.heap_w[hp.heap_w[i]] 相当于 dist[i].
    int i, j, k;
    int t;
    int64 temp;
    for (t = 1; t < n; t++)
    {
        i = hp.heap_v[1];
        temp = hp.heap_w[1]; // temp 相当于dist[i].
        hp.Delmin();
        visited[i] = 1;
        if (visited[dt]) break; // 说明dist[dt]已经求出.
        for (k = first[i]; k != -1; k = edge[k].next)
        {
            j = edge[k].b;
            if (!visited[j] && temp + edge[k].c < hp.heap_w[hp.heap_map[j]])
            {
                hp.heap_w[hp.heap_map[j]] = temp + edge[k].c;
                hp.heap_up(hp.heap_map[j]);
            }
        }
    }
}

int main(void)
{
//    freopen("Input.txt", "r", stdin);
    int i, j;
    int a, b, c;
    while (scanf("%d %d %d %d", &n, &m, &ds, &dt) != EOF)
    {
        Init();
        while (m--)
        {
            scanf("%d %d %d", &a, &b, &c);
            AddEdge(a, b, c);
//            AddEdge(b, a, c); // 无向图.
        }
        Dijkstra_heap();
        ans = hp.heap_w[hp.heap_map[dt]];
        if (ans == INF) cout << "Can't reach" << endl;
        else cout << ans << endl;
    }
    return 0;
}