while((*gets(buffer) !='\n') && (i<NUM_P))
{
pS[i]=(char *)malloc(strlen(buffer)+1);

if(pS[i]==NULL)                            /* Check for no memory allocated */
{
      printf("Memory allocation failed.Program terminated.\n");
      return;
}
strcpy(pS[i++],buffer);


}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define  BUFFER_LEN  100                    /* Length of input buffer */
#define  NUM_P  100                         /* maximum number of string */
#define  TRUE  1
#define  FALSE 0

void main()
{
char buffer[BUFFER_LEN];                                /*Store for string */
char *pS[NUM_P]= { NULL };                                   /*Array of string pointers */
char *pTemp = NULL;                                 /* Pointer to buffer */
int i;                                                  /* Loop counter */
int sorted = FALSE;
int last_string = 0;                                                /* store the final i */

/*read the string from the keyboard */

printf("\nEnter successive lines,pressing Enter at the end of each line"
                   ".\n Just press Enter to end.\n\n");

while((*gets(buffer) !='\n') && (i<NUM_P))
{
pS[i]=(char *)malloc(strlen(buffer)+1);

if(pS[i]==NULL)                            /* Check for no memory allocated */
{
      printf("Memory allocation failed.Program terminated.\n");
      return;
}
strcpy(pS[i++],buffer);
}
last_string=i;
}

你没有对变量i初始化就使用，造成内存写入错误。调试时pS[i]=(char *)malloc(strlen(buffer)+1);i=-858993460,所以pS[i]=ps[-858993460]。

哦哦  确实忘记初始化了  把i=0；后   程序运行正常了  谢谢啦