Reverse order of words in a string

Question

I am working a problem to reverse the order of words in a string. For example, if you have str1 = "the sky is blue" then the solution should be "blue is sky the".

Here is my code:

class Solution 
{
    public:
        list<string> words;

        void createList(string s)
        {
            istringstream iss(s);
            string token;

            while(getline(iss, token, ' '))
            {
                words.push_back(token);
            }
        }

        string reverseWords(string s) 
        {
            list<string>::iterator iter = words.begin();
            string newString = "";

            createList(s);
            newString.append(*iter);
            for (iter = (iter+1); iter != words.end(); iter++)
            {
                newString.append(" ");
                newString.append(*iter);
            }

            return newString;
        }
};

My question is.... Am I using the list iterator correctly? I got a compiler error that said "Line 25: no match for 'operator+' " referring to the for loop in reverseWords().

Answer 1

list does not support random access iterators, therefore using operator+ with list is not allowed, you would use operator++. You have to access the list elements sequentially, just like you'd have to if you were using your own linked list implementation.

You could use a reverse iterator, to reverse your string.

#include <list>
#include <string>
#include <iostream>
#include <sstream>

using namespace std;

class Solution 
{
    public:
        list<string> words;

        void createList(string& s)
        {
            istringstream iss(s);
            string token;

            while(getline(iss, token, ' '))
            {
                words.push_back(token);
            }
        }

        string reverseWords(string& s) 
        {
            list<string>::reverse_iterator iter = words.rbegin();
            string newString = "";

            createList(s);
            for ( ; iter != words.rend(); ++iter)
            {
                newString.append(" ");
                newString.append(*iter);
            }

            return newString;
        }
};

int main(int, char**)
{
    string in("The sky is always blue");
    Solution s;
    string out = s.reverseWords(in);

    std::cout << in << std::endl;
    std::cout << out << std::endl;
}

Answer 2

this compiles fine, so yes the ++iter is a valid statement. Incrementing is valid, jumping is not:

list<string> words;

void createList(string s)
{
    istringstream iss(s);
    string token;

    while(getline(iss, token, ' '))
    {
        words.push_back(token);
    }
}

string reverseWords(string s) 
{
    list<string>::iterator iter = words.begin();
    string newString = "";

    createList(s);
    newString.append(*iter);
    ++iter;
    for (; iter != words.end(); ++iter)
    {
        newString.append(" ");
        newString.append(*iter);
    }

    return newString;
}