[c-prog] question about compiler warning

[frank zhu]

Hi,
I've coded a program to find the nth node from the end of the linked list.
It works and get me the right result, but I am puzzled as why I get the 
compiler warnings at the following line,
p1 = p1->next;
findnthnode.c:28: warning: assignment from incompatible pointer type

Thanks in advance,


////////////////  Code listing below /////////////////////////

//Q: In a linked list find the nth node from the end of this list.

#include <stdio.h>
// algorithm : use two pointers and they're n-1 nodes apart. when first 
reach the end, 2nd one reaches nth node.

typedef struct {
    struct node* next;
    int data;
}node;

node* findnthnode(node *head, int n)
{
    node *p1;
    node *p2;
    p1 = head;
    p2 = NULL;

    // if head is null, return null
    // if n > length of list, return null
    // if n = 0, we return the tail.
    // if there're two nodes,
    // n=0, we return node2,
    // n=1, we return node1(head)
    // n>=2, we return NULL
   
    // let p1 go n-1 steps
    for (; n>0 && p1 ; n--){
        p1 = p1->next;
    }

    if (!p1)
        return NULL;
    else
        p2 = head;

    // move p1, p2 together to traverse list
    while (p1 && p1->next){
        p1 = p1->next;
        p2 = p2->next;
    }
    return p2;
}

int main()
{
    // build a list
    node a[3] = {0};
    node *p;
    int i;
    a[0].data = 2;
    a[0].next = &a[1];
    a[1].data = 5;
    a[1].next = &a[2];
    a[2].data = 3;
    a[2].next = NULL;

    for (i=0;i<4;i++){
        p = findnthnode(a, i);
        if (p)
            printf("found %dth node =  %d\n",i, p->data);
        else
            printf("not found!\n");
    }
    return 0;
}