Bellman_Ford算法

      Bellman_Ford算法和Dijkstra算法都可以用来求解有向图的单源最短路径问题，但是，相比于Dijkstra算法， Bellman_Ford算法允许边的权重为负值。

 

    算法的详细讨论见算法导论或者下面这个博客http://blog.csdn.net/niushuai666/article/details/6791765

 

  代码如下：

#include<iostream>

using namespace std;

#define Inf 65535
#define NotAVerter -1

/////////////////////邻接链表的相关定义//////////////////////
typedef struct EdgeNode *position;
typedef struct Led_table* Table;


struct EdgeNode     //边表结点
{
	int adjvex;    // 邻接点域,存储该顶点对应的下标
	int weight;     // 对应边的权值
	int dis;     //此数据记录从源点到该节点的最短距离
	int precursor;  //此数据记录该节点在广度优先树种的前驱节点
	position next; // 链域,指向下一个邻接点
};

struct Led_table       // 邻接表结构
{
	int data;                //邻接表的大小
	position *firstedge;       //边表头指针，可以理解为数组
};


//////////////////////////邻接链表相关函数定义///////////////
Table Creat_Lable(int MaxElements)    //MaxElements参数为希望创建的节点数
{

	Table table1 = static_cast<Table> (malloc(sizeof(struct Led_table)));
	table1->data = MaxElements;
	if (table1 == NULL)
	{
		cout << "out of space!!!";
	}

	table1->firstedge = static_cast<position*>(malloc(sizeof(position)*(table1->data)));
	if (table1->firstedge == NULL)
	{
		cout << "out of space!!!";
	}

	//给每个表头赋值，从0开始
	for (int i = 0; i <= table1->data - 1; ++i)
	{
		table1->firstedge[i] = static_cast<position>(malloc(sizeof(EdgeNode)));   //申请一个节点
		if (table1->firstedge[i] == NULL)
		{
			cout << "out of space!!!";
		}
		table1->firstedge[i]->adjvex = 0;   //表头这个参数没有意义
		table1->firstedge[i]->weight = 0;   //表头这个参数没有意义
		table1->firstedge[i]->dis = Inf;
		table1->firstedge[i]->precursor = NotAVerter;
		table1->firstedge[i]->next = NULL;

	}
	return table1;

}


void Insert(Table table1, int v, int w, int weig)   //表示存在一条边为<v,w>
{
	position p = static_cast<position>(malloc(sizeof(EdgeNode)));   //申请一个节点
	if (p == NULL)
	{
		cout << "out of space!!!";
	}
	p->adjvex = w;
	p->weight = weig;    //对于无权图来说，该域可以设置为1
	p->dis = Inf;    //对于普通节点来说无意义
	p->precursor = NotAVerter;  //对于普通节点来说无意义
	p->next = table1->firstedge[v]->next;
	table1->firstedge[v]->next = p;

}

void init_yuandian(Table table1, int s)  //把s设置为图的源点
{
	table1->firstedge[s]->adjvex = 0;
	table1->firstedge[s]->weight = 0;
	table1->firstedge[s]->dis = 0;    //源点的这个值设置为0
	table1->firstedge[s]->precursor = NotAVerter;
}


bool Bellman_Ford(Table table1, int s)
{
	for (int i = 1; i <= table1->data - 1; ++i)   //每条边进行N-1次松弛操作
	{
		for (int j = 0; j <= table1->data - 1; ++j)  //对每条边
		{
			position p = table1->firstedge[j]->next;
			while (p != NULL)
			{
				if (table1->firstedge[p->adjvex]->dis > table1->firstedge[j]->dis + p->weight)   //松弛操作
				{
					table1->firstedge[p->adjvex]->dis = table1->firstedge[j]->dis + p->weight;
					table1->firstedge[p->adjvex]->precursor = j;
				}
				p = p->next;
			}
		}
	}

	bool flag = true;
	for (int j = 0; j <= table1->data - 1; ++j)  //对每条边
	{
		position p = table1->firstedge[j]->next;
		while (p != NULL)
		{
			if (table1->firstedge[p->adjvex]->dis > table1->firstedge[j]->dis + p->weight)
			{
				flag = false;
				break;
			}
			p = p->next;
		}
	}
	return flag;
}

void print_path(Table table1, int v)
{
	if (table1->firstedge[v]->precursor != NotAVerter)
		print_path(table1, table1->firstedge[v]->precursor);
	cout << "v" << v << endl;
}




int main()
{
	Table table_1 = Creat_Lable(5);    //创建一个大小为5的邻接表

	Insert(table_1, 0, 1, 6); Insert(table_1, 0, 3, 7);
	Insert(table_1, 1, 2, 5); Insert(table_1, 1, 3, 8); Insert(table_1, 1, 4, -4);
	Insert(table_1, 2, 1, -2);
	Insert(table_1, 3, 2, -3); Insert(table_1, 3, 4, 9);
	Insert(table_1, 4, 0, 2); Insert(table_1, 4, 2, 7); 

	init_yuandian(table_1, 0);  //把0设置为图的源点
	cout << Bellman_Ford(table_1, 0) << endl;

	cout << table_1->firstedge[4]->dis << endl;

	print_path(table_1, 4);


	return 0;
}

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