Read the How to prepare for an interview series at my blog: http://se7so.blogspot.com/2014/01/how-to-prepare-for-interview.html

prob3.cpp

#include <iostream>
#include <cstring>
using namespace std;

string removeDuplicates(string s) {
	bool chs[512];
	memset(chs, false, sizeof chs);
	
	int j = 0;
	for(int i = 0; i < s.size(); i++) {
		if(!chs[s[i]]) {
			s[j++] = s[i];
			chs[s[i]] = true;
		}
	}
	
	return s.substr(0, j);
}

int main() {
	
	string s = "aaaabaaaacbbbbd";
	s = removeDuplicates(s);
	cout << s << endl;
	
	cout << removeDuplicates("a") << endl;
	cout << removeDuplicates("") << endl;
	cout << removeDuplicates("aaaaaaaaaaaaaaa") << endl;
	cout << removeDuplicates("aaaaaaaaaabbbbbbbbbbbbcccccccccc") << endl;
	
	return 0;
}