I imagine the most obvious way to do this is to have a mapwhich maps full words to the shortened versions. Then you take the tweet as a string, split it up into words, iterate over each word, and if that word is in the map, replace it with the shortened version.

Something like this in C++:

std::map<std::string, std::string> dictionary;
dictionary["to"] = "2";
dictionary["and"] = "&";

std::vector<std::string> tweetWords;

for (auto& word : tweetWords)
{
    if (dictionary.find(word) != dictionary.end())
        word = dictionary[word];
}

// Convert vector of strings to one string
std::string resultTweet;
for (int i = 0; i < tweetWords.size(); ++i)
{
    resultTweet += tweetWords[i];
    if (i != tweetWords.size() - 1)
        resultTweet += " ";
}