/* 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;}