Fast algorithm C code for full letter arrangement

All arranged, such as ABC of letters, all arranged with a, AB,AC,ABC,ACB,B,BA,BC,BAC,BCA,C,CA,CB,CAB,CBA.

The principle is to insert new characters in all positions of a string.
such as: AB insert C, position has 1a2b3, insert after the formation of CAB ACB ABC
Char *alllist (char *str, int *pnum)
... {
int I, j, K, N;
int len = strlen (str);
int total = 0;
int count, Oldcount;
int size;
Char *buf;
Char *p, *P1;
if (Len >) return NULL;
Calculates the total number of combinations
for (i = 0, j = 1; i < Len; i++)
... {
J *= (Len-i);
Total = j;
}
Create a two-dimensional array that holds all the combinations
size = len + 1;
if ((Buf = (char *) malloc (total * size) = = NULL)
... {
return NULL;
}
for (k = 0, count = 0; k < len; k++)//all characters to insert
... {
Oldcount = count;
p = Buf;
P1 = Buf + count * size;
for (i = 0; i < oldcount; i++, p + = size)//INSERT into all strings to form a new string
... {
n = strlen (p);
for (j = 0; J <= N; j + +, count++, p1 = size)//Insert all positions in string
... {
memcpy (P1, p, N);
P1[n] = P1[j];
P1[J] = str[k];
P1[n + 1] = '

Arrange 10 letters on my machine for about 0.5 seconds.