Remove Duplicates from an Unsorted Sring

PROBLEM :

Given a string, remove duplicates from it. Note that original order of characters must be kept same.  Expected time complexity O(n) where n is length of input string and extra space O(1) under the assumption that there are total 256 possible characters in a string.

Input: First line of the input is the number of test cases T. And first line of test case contains a string.

Output:  Modified string without duplicates and same order of characters.

Constraints:  Input String Length <= 1000

Example:
Input:
2
geeksforgeeks
geeks for geeks

Output:
geksfor
geks for
     
--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION :
--------------------------------------------------------------------------------

#include<iostream>
using namespace std;
#include<string.h>
char * Remove_Duplicates(char *) ;
int main()
 {
int t ;
char *str ;
str=(char*)malloc(1000*sizeof(char)) ;
cin>>t ;
while(t--)
{
   scanf(" %[^\n]s",str) ;
 
   str=Remove_Duplicates(str) ;
   cout<<str<<endl ;
}
return 0;
}

char * Remove_Duplicates(char *str)
{
int hash[256]={0} ;
    int i,l,k ;
    l=strlen(str) ;
    k=0 ;
   
    for(i=0;i<l;i++)
        hash[str[i]]=1 ;
       
    for(i=0;i<l;i++)
    {
        if(hash[str[i]]==1)
        {
            str[k]=str[i] ;
            k++ ;
            hash[str[i]]=0 ;
        }
    }
    str[k]='\0' ;
    return str ;
}

---------------------------------------------------------------------------------