Thoroughly understand why the list uses a two-level pointer or a first-level pointer reference __ data structure and algorithm

Reprint: http://blog.csdn.net/u012434102/article/details/44886339

When you write data structures with C + +, the link list and two-fork tree often need to use a two-level pointer or a pointer to a reference, then when to use when not.
First look at a simple C + + list operating procedures:

 #include "stdio.h" #include "stdlib.h" #include "time.h" #define OK 1 #define ERROR 0 #define TRUE 1  #define FALSE 0 #define MAXSIZE 20/* Storage space initial allocation/typedef int STATUS;/* Status is the type of function whose value is the function result status code, such as OK/typedef  
    The int elemtype;/* elemtype type is based on the actual situation, which is assumed to be int */Status visit (elemtype c) {printf ("%d", c);  
return OK;  
    typedef struct NODE {elemtype data;  
struct Node *next;  
}node; typedef struct NODE *linklist; /* Define LINKLIST///Initialize header, use first level pointer (this method is invalid) Status InitList1 (linklist L)//equivalent to node *l {l= (linklist) malloc (size Of (Node)); /* Generate head node, and make L point to this head node * * * IF (!  
    L)/* Storage allocation failure/return ERROR; l->next=null;  
/* Pointer field is empty/return OK; ///Initialize header, with level two pointer Status InitList2 (linklist *l)//equivalent to Node **l {*l= (linklist) malloc (sizeof (node));/* Generating Header node, and make L point to this head node/if (!  
    *L)/* Storage allocation failure/return ERROR; (*l)->next=null;  
/* Pointer field is empty/return OK;  

}Initializes the header with a level pointer referencing the Status InitList3 (linklist &l)//equivalent to Node *&l {l= (linklist) malloc (sizeof (node));/* Generated Head node and make L point to this head node * * * IF (!  
    L)/* Storage allocation failure/return ERROR; l->next=null;  
/* Pointer field is empty/return OK;  
    }//Empty list, use level two pointer Status ClearList1 (linklist *l) {linklist p,q;           p= (*l)->next;  
        /* p points to the first node/while (p)/* not to the footer/{q=p->next;  
        Free (p);  
    p=q;        } (*l)->next=null;  
/* head node pointer field is empty/return OK;  
    //Empty the list, using the first level pointer Status ClearList2 (linklist L) {linklist p,q;           p=l->next;  
        /* p points to the first node/while (p)/* not to the footer/{q=p->next;  
        Free (p);  
    p=q;        } l->next=null;  
/* head node pointer field is empty/return OK;  
    }//Destroy linked list, use first level pointer (this method is invalid) Status DestroyList1 (linklist L) {linklist p,q;           p=l->next; /* p points to the first node * *(p)/* not to the end of the table * * q=p->next;  
        Free (p);  
    p=q;  
    Free (L);  
    L=null;  
return OK;  
    }//Destroy linked list, use level two pointer Status DestroyList2 (linklist *l) {linklist p,q;           p= (*l)->next;  
        /* p points to the first node/while (p)/* not to the footer/{q=p->next;  
        Free (p);  
    p=q;  
    Free (*l);  
    *l=null;  
return OK;  
    }//Destroy linked list, use the first level pointer to reference Status DESTROYLIST3 (linklist &l) {linklist p,q;           p=l->next;  
        /* p points to the first node/while (p)/* not to the footer/{q=p->next;  
        Free (p);  
    p=q;  
    Free (L);  
    L=null;  
return OK;   
    }/* Initial condition: sequential linear table L already exist, 1≤i≤listlength (L)////////////////////////////Status Getelem (linklist l,int i,elemtype *e) {  
    Int J;     Linklist p;     /* Declare a node p * * * p = l->next;      /* Let P point to the first node of the linked list L/j = 1; /* J for counter */While(P && j<i)/* p is not null or the counter J is not equal to I, the loop continues * * p = p->next;  
    /* Let P point to the next node * * ++J;  } if (!p | | j>i) return ERROR;   /* I element does not exist */*e = p->data;  
* * Take the first element of the data/return OK;  
    ///Insert element in the middle, with level two pointer Status ListInsert1 (linklist *l,int i,elemtype e) {int J;  
    Linklist p,s;     
    p = *l;  
    j = 1;  
        while (P && J < i) * * Find the first node/{p = p->next;  
    ++j;   } if (!p | | j > i) return ERROR;  /* I element does not exist/s = (linklist) malloc (sizeof (Node));    
    /* Generate new node (C language standard function) */s->data = e;      S->next = p->next;          /* Assign the subsequent node of P to the successor of S/p->next = s;  
/* Assign value to P's successor/return OK;  
    }//Insert element in the middle, with the first level pointer Status ListInsert2 (linklist l,int i,elemtype e) {int J;  
    Linklist p,s;     
    p = L;  
    j = 1;  
       while (P && J < i) * * Find the first node/{ p = p->next;  
    ++j;   } if (!p | | j > i) return ERROR;  /* I element does not exist/s = (linklist) malloc (sizeof (Node));    
    /* Generate new node (C language standard function) */s->data = e;      S->next = p->next;          /* Assign the subsequent node of P to the successor of S/p->next = s;  
/* Assign value to P's successor/return OK;  
    ///delete an element with the level two pointer Status ListDelete1 (linklist *l,int i,elemtype *e) {int J;  
    Linklist p,q;  
    p = *l;  
    j = 1;  
        while (P->next && J < i)/* traversal look for the first element/{p = p->next;  
    ++j; } if (! ( P->next) | |           J > I) return ERROR;  
    /* I element does not exist */q = p->next;            P->next = q->next;               /* Assign the successor of Q to P's successor/*e = q->data;                    * * to the Q node in the data to E/free (q);  
/* Let the system recycle this node, free memory/return OK;  
    ///delete an element with the first level pointer Status ListDelete2 (linklist l,int i,elemtype *e) {int J;  
    Linklist p,q;  
 p = L;   j = 1;  
        while (P->next && J < i)/* traversal look for the first element/{p = p->next;  
    ++j; } if (! ( P->next) | |           J > I) return ERROR;  
    /* I element does not exist */q = p->next;            P->next = q->next;               /* Assign the successor of Q to P's successor/*e = q->data;                    * * to the Q node in the data to E/free (q);  
/* Let the system recycle this node, free memory/return OK;  
        while (p) {visit (p->data);  
    p=p->next;  
    printf ("\ n");  
return OK;  
    int main () {linklist L;  
    Elemtype e;  
    Status i;  
    int j,k;   InitList1 (L);  First-level pointer method to create the table header, Failure//initlist2 (&AMP;L);     Second-level pointer method to create a table header, successful InitList3 (L);  
    First-level pointer reference to create a table header, success for (j=1;j<=7;j++) ListInsert2 (L,1,J);  
    printf ("First-level pointer is inserted after 1~7 in the header of L"); ListtrAverse (L);  
    ListInsert1 (&l,3,12);  
    printf ("Two-level pointer mode inserted in the middle of L 12 after:");   

    Listtraverse (L); ListInsert2 (L,