Prefix to Postfix expression

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

#define MAX 100

/*
author              :sailesh Kumar
Website             :www.yongsters.co.cc
Organisation        :saileshkumar.org

*/

void convert(char prefix[], char postfix[]);
int find(char str[]);
char* substr(char src[], int offset, int len, char dest[]);

int main()
{
 char prefix[MAX];
 char postfix[MAX];
 printf("enter the prefix string\n");
 scanf(" %s", prefix);
 convert(prefix, postfix);
 printf("%s\n", postfix);

 return 0;
}

//algo ::
//1.If the prefix string is a single variable, it is its own postfix equivalent
//2.Let op be the first operator of the prefix string
//3.Find the first operand, opnd1 of the string.Convert it to postfix and call it post1.
//4.Find the second operand, opnd2, of the string.Convert it to postfix and cal it post2.
//5.Concatenate post1, post2, and op.
void convert(char prefix[], char postfix[])
{
 char opnd1[MAX];
 char opnd2[MAX];
 char post1[MAX];
 char post2[MAX];
 char temp[MAX];
 char op[2];
 int length;
 int i,j,m,n;

 //prefix string is single variable
 if((length=strlen(prefix))==1){
  if(isalpha(prefix[0])){
   postfix[0]=prefix[0];
   postfix[1]='\0';
   return;
  }

  printf("illegal prefix string\n");
  exit(1);
 }

 //prefix string is longer
 op[0]=prefix[0];
 op[1]='\0';
 substr(prefix,1,length-1,temp);
 m=find(temp);
 substr(prefix, m+1, length-m-1, temp);
 n=find(temp);

 if((op[0]!='+' && op[0]!='-' && op[0]!='*' && op[0]!='/') || (m==0) || (n==0) || (m+n+1!=length)){
  printf("illegal prefix string\n");
  exit(1);
 }
 substr(prefix, 1, m, opnd1);
 substr(prefix, m+1, n, opnd2);

 convert(opnd1, post1);
 convert(opnd2, post2);
 strcat(post1, post2);
 strcat(post1, op);

 substr(post1, 0, length, postfix);
}

int find(char str[])
{
 char temp[MAX];
 int length;
 int i,j,m,n;

 if((length=strlen(str))==0) return 0;

 //first character is a letter
 if(isalpha(str[0])!=0)  return 1;

 //otherwise find the first operand
 if(strlen(str)<2) return 0;

 substr(str, 1, length-1, temp);
 m=find(temp);

 //no valid prefi operand or no second operand
 if(m==0 || strlen(str)==m) return 0;

 substr(str, m+1, length-m-1, temp);
 n=find(temp);

 if(n==0) return 0;

 return (m+n+1);
}

char* substr(char src[], int offset, int len, char dest[])
{
 int i;
 for(i = 0; i < len && src[offset + i] != '\0'; i++){
  dest[i] = src[i + offset];
 }
 dest[i] = '\0';

 return dest;
}