Decv++调试报错 program received signal SIGSEGV,Segmentation fault

小白写数据结构中 用邻接表表示图并输出中 在调试界面出现了问题
program received signal SIGSEGV,Segmentation fault
并不是很懂

#include<iostream>
#include<stdio.h>
#define MVNum 100                           
typedef struct ArcNode  //边节点 
{                       
    int adjvex;                                 
    struct ArcNode * nextarc;                                                  
}ArcNode;

typedef struct VNode    //顶点节点 
{ 
    char data;                       
    ArcNode * firstarc;                 
}VNode, AdjList[MVNum];              

typedef struct                 //邻接表 
{ 
    AdjList vertices;                       
    int vexnum, arcnum;                     
}ALGraph; 


int LocateVex(ALGraph N,char v)
{
    int i;
    for(i=0;i<N.vexnum;i++)
    {
        if(N.vertices[i].data==v)
        return i;
    }
}


int CreateUDG(ALGraph &G)
{
    int i,j,k,v1,v2;
    ArcNode *p1,*p2;
    std::cin>>G.vexnum>>G.arcnum;
    for(i=0;i<G.vexnum;i++)
    {
    std::cin>>G.vertices[i].data;
    G.vertices[i].firstarc=NULL;  
    }//顶点节点（4个）全部排在了数组里面； 
    for(k=0;k<G.arcnum;k++)
    {
       std::cin>>v1>>v2;
       i=LocateVex(G,v1);
       j=LocateVex(G,v2);
       p1=new ArcNode;
       p1->adjvex=j;
       p1->nextarc=G.vertices[i].firstarc;
       G.vertices[i].firstarc=p1;
       p2->adjvex=i;
       p2->nextarc=G.vertices[j].firstarc;
       G.vertices[j].firstarc=p1;
    }   
}

int main()
{
    int i;
    ALGraph G;
    ALGraph *p;
    ArcNode *pp; 
    CreateUDG(G);
    for(i=0;i<G.vexnum;i++)
    {
        std::cout<<p->vertices[i].data;
        std::cout<<"->";
        pp=p->vertices[i].firstarc;
        do
        {
        std::cout<<pp->adjvex;
        pp=pp->nextarc;
        }while(pp==NULL);
    printf("\n");
    }

}