初学者，空指针，邻接表

菜鸟求助，空指针，邻接表

Java code

package examples;import adjlist.AdjList;import adjlist.OpAdjList;public class Example7_9 {public static void main(String[] args) {        AdjList g;                            int p;        int i;        int m[][] = {{0,1,1,0,0},{1,0,0,1,1},{1,0,0,0,0},{0,1,0,0,1},{0,1,0,1,0}};        g = OpAdjList.CreateGraph(m, 5);        System.out.printf("\n1:\n");        OpAdjList.DispGraph(g);                            System.out.printf("\n\n\n2:\n");        for(i=0;i<g.vexs;i++)        {            p=OpAdjList.GetFirst(g,i);            if(p!=-1)                    System.out.printf("\nLine%d\t[%d]",i,p);            else                    System.out.printf("\nLine%d",i);            while(p!=-1)            {                p=OpAdjList.GetNext(g,i,p);                 if(p!=-1)                        System.out.printf("\t[%d]",p);            }        }    }}

Java code

package adjlist;/* 邻接表类型 */public class AdjList {    public static final int MAXVEX = 50;    /* 顶点表 */    public VexNode lists[] = new VexNode[MAXVEX ];    /* 边数，顶点数 */    public int edges, vexs;}/* 边表结点类型 */class ArcNode {    /* 顶点序号 */    int adjvex = 0;    /* 指向下一个邻接点 */    ArcNode next;}/* 顶点表结点类型 */class VexNode {    /* 顶点信息 */    int data;    /* 边表头指针 */    ArcNode head ;}

Java code

package adjlist;public class OpAdjList {/*    public static AdjList CreateGraph(int m[][], int n) {        return CreateGraph(m, n);    }*/    public static boolean DispGraph(AdjList g) {        return DispGraph1(g);    }    public static int GetFirst(AdjList g, int k) {        return GetFirst(g, k);    }    public static int GetNext(AdjList g, int k, int u) {        return  GetNext1(g, k, u) ;    }    /* 算法7-9创建邻接表 */public static AdjList CreateGraph(int m[][], int n) {        int i, j;        ArcNode p;        AdjList g = new AdjList();        g.edges = 0;        g.vexs = n;        for (i = 0; i < n; i++) {            g.lists[i].head = null;            for (j = n - 1; j >= 0; j--) {                if (m[i][j] != 0) {                    p = new ArcNode();                    p.adjvex = j;                    p.next = g.lists[i].head;                    g.lists[i].head = p;                    g.edges++;                }            }        }        return g;    }    /* 算法7-10显示邻接表 */    private static boolean DispGraph1(AdjList g) {        int i;        ArcNode p;        for (i = 0; i < g.vexs; i++) {            System.out.printf("\nLine%d:\t", i);            p = g.lists[i].head;            while (p != null) {                System.out.printf("[%d]\t", p.adjvex);                p = p.next;            }        }        return true;    }    /* 算法7-11第一个邻接点 */    private static int GetFirst1(AdjList g, int k) {        if (k < 0 || k > g.vexs) {            System.out.printf("参数k超出范围！\n");            return -1;        }        if (g.lists[k].head == null)            return -1;        else            return g.lists[k].head.adjvex;    }    /* 算法7-12下一个邻接点 */    private static int GetNext1(AdjList g, int k, int u) {        ArcNode p;        if (k < 0 || k > g.vexs || u < 0 || u > g.vexs) {            System.out.printf("参数k或u超出范围！\n");            return -1;        }        p = g.lists[k].head;        while (p.next != null && p.adjvex != u)            p = p.next;        if (p.next == null)            return -1;        else            return p.next.adjvex;    }}

这句：

Java code

g.lists[i].head = null;