SPOJ 15. The Shortest Path 最短路径题解

本题就是给出一组cities。然后以下会询问，两个cities之间的最短路径。

属于反复询问的问题，临时我仅仅想到使用Dijsktra+heap实现了。

由于本题反复查询次数也不多，故此假设保存全部最短路径，那么是得不偿失了。

所以还是反复使用Dijsktra吧。

有没有更加好的办法处理反复查询问题呢？还没想到。

本算法纯粹手工打造了，不使用stl。代码非常长非常长，光打一遍就会手软的，呵呵。

原题：

You are given a list of cities. Each direct connection between two cities has its transportation cost (an integer bigger than 0). The goal is to find the paths of minimum cost between pairs of cities. Assume that the cost of each path (which is the sum of costs of all direct connections belongning to this path) is at most 200000. The name of a city is a string containing characters a,...,z and is at most 10 characters long.

Input

s [the number of tests <= 10]
n [the number of cities <= 10000]
NAME [city name]
p [the number of neighbours of city NAME]
nr cost [nr - index of a city connected to NAME (the index of the first city is 1)]
           [cost - the transportation cost]
r [the number of paths to find <= 100]
NAME1 NAME2 [NAME1 - source, NAME2 - destination]
[empty line separating the tests]

Output

cost [the minimum transportation cost from city NAME1 to city NAME2 (one per line)]

Example

Input:
1
4
gdansk
2
2 1
3 3
bydgoszcz
3
1 1
3 1
4 4
torun
3
1 3
2 1
4 1
warszawa
2
2 4
3 1
2
gdansk warszawa
bydgoszcz warszawa

Output:
3
2

#pragma once
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <string>
#include <map>
using namespace std;

class TheShortestPath15
{
	struct Node
	{
		int des, weight;
		Node *next;
		Node(int d, int w) : des(d), weight(w), next(NULL) {}
	};

	struct AdjList
	{
		Node *head;
		AdjList() : head(NULL) {}
	};

	struct Graph
	{
		int v;
		AdjList *arr;
		Graph(int v1) : v(v1)
		{
			arr = new AdjList[v];
		}
		~Graph()
		{
			for (int i = 0; i < v; i++)
			{
				Node *h = arr[i].head;
				while (h)
				{
					Node *next = h->next;
					delete h, h = NULL;
					h = next;
				}
			}
			delete arr, arr = NULL;
		}
	};

	void addEdge(Graph *gra, int src, int des, int w)
	{
		Node *n = new Node(des, w);
		n->next = gra->arr[src].head;
		gra->arr[src].head = n;
		/*
		n = new Node(src, w);
		n->next = gra->arr[des].head;
		gra->arr[des].head = n;
		*/
	}

	struct HeapNode
	{
		int v, dist;
		explicit HeapNode(int v1, int d) : v(v1), dist(d) {}
	};

	struct Heap
	{
		int size, cap;
		int *pos;
		HeapNode **arr;
		Heap(int c) : cap(c), size(0)
		{
			pos = new int[c];
			arr = new HeapNode*[c];
		}
		~Heap()
		{
			delete [] pos, pos = NULL;
			for (int i = 0; i < size; i++)
			{
				if (arr[i]) delete arr[i], arr[i] = NULL;
			}
			delete [] arr;
		}
	};

	void swapHeapNodes(HeapNode **a, HeapNode **b)
	{
		HeapNode *c = *a;
		*a = *b;
		*b = c;
	}

	void heapify(Heap *heap, int node)
	{
		if (!heap) return ;
		int minN = node;
		int left = (node<<1) + 1;
		int right = (node<<1) + 2;

		if (left < heap->size &&
			heap->arr[left]->dist < heap->arr[minN]->dist) minN = left;
		
		if (right < heap->size &&
			heap->arr[right]->dist < heap->arr[minN]->dist) minN = right;

		if (minN != node)
		{
			heap->pos[heap->arr[minN]->v] = node;
			heap->pos[heap->arr[node]->v] = minN;

			swapHeapNodes(&heap->arr[minN], &heap->arr[node]);

			heapify(heap, minN);
		}
	}

	inline bool isEmpty(Heap *heap)
	{
		return heap->size == 0;
	}

	HeapNode *extraMin(Heap *heap)
	{
		if (isEmpty(heap)) return NULL;

		HeapNode *root = heap->arr[0];
		HeapNode *last = heap->arr[heap->size-1];
		heap->arr[0] = last;//别漏了这步。


		heap->pos[root->v] = heap->size-1;
		heap->pos[last->v] = 0;

		--heap->size; //别忘记先--
		heapify(heap, 0);

		return root;
	}

	void decreaseKey(Heap *heap, int v, int dist)
	{
		int i = heap->pos[v];

		heap->arr[i]->dist = dist;

		while (i && heap->arr[i]->dist < heap->arr[(i-1)>>1]->dist)
		{
			heap->pos[heap->arr[i]->v] = (i-1)>>1;
			heap->pos[heap->arr[(i-1)>>1]->v] = i;

			swapHeapNodes(&heap->arr[i], &heap->arr[(i-1)>>1]);

			i = (i-1)>>1;
		}
	}

	inline bool isInHeap(Heap *heap, int v)
	{
		return heap->pos[v] < heap->size;
	}

	void dijsktra(Graph *gra, int src, int des, int dist[])
	{
		Heap *heap = new Heap(gra->v);
		heap->size = gra->v;

		for (int i = 0; i < gra->v; i++)
		{
			dist[i] = INT_MAX;
			heap->pos[i] = i;
			heap->arr[i] = new HeapNode(i, dist[i]);
		}

		dist[src] = 0;
		decreaseKey(heap, src, 0);

		while (!isEmpty(heap))
		{
			HeapNode *hn = extraMin(heap);
			int u = hn->v;
			delete hn, hn = NULL;

			if (u == des) break;		//这里添加代码。仅仅找到目标节点就可返回了
			if (dist[u] == INT_MAX) break;

			Node *n = gra->arr[u].head;
			while (n)
			{
				if (isInHeap(heap, n->des) && 
					n->weight + dist[u] < dist[n->des])
				{
					dist[n->des] = n->weight + dist[u];
					decreaseKey(heap, n->des, dist[n->des]);
				}
				n = n->next;
			}
		}
		delete heap;
	}

public:
	TheShortestPath15()
	{
		int s, n, p, nr, cost, r;
		map<string, int> cities;
		string name;
		scanf("%d", &s);
		while (s--)
		{
			scanf("%d", &n);
			Graph *gra = new Graph(n);
			for (int i = 0; i < n; i++)
			{
				//gets(NAME);教训：gets是取到
或者EOF结束的，不是取单个单词
				cin>>name;
				cities[name] = i;

				scanf("%d", &p);
				while (p--)
				{
					scanf("%d %d", &nr, &cost);
					addEdge(gra, i, nr-1, cost);
				}
			}
			scanf("%d", &r);
			while (r--)
			{
				cin>>name;
				int src = cities[name];

				cin>>name;
				int des = cities[name];

				int *dist = (int *) malloc(sizeof(int) * n);
				dijsktra(gra, src, des, dist);
				printf("%d
", dist[des]);
				if (dist) free(dist);
			}
			delete gra;
		}
	}
};