Details: http://codingdragon.tumblr.com/post/66782834831/longest-2-character-substring

maxLenSubstring2Chars.cpp

pair<pair<char,char>,int> maxLenSubstring2Chars(string s){
    pair<char,int> lastChar;
    lastChar.first=s[0];
    lastChar.second=1;
    
    pair<pair<char,char>,int> longestPairYet;
    longestPairYet.second=-1;
    
    pair<pair<char,char>,int> currentPair;
    currentPair.first.first=s[0];
    currentPair.second=-1;
    
    for(int i=1;i<=s.length();i++){
        
        //Check if current char present in the current pair
        if(s[i]==currentPair.first.first || s[i]==currentPair.first.second) {
            if(currentPair.second!=-1) currentPair.second++;
            
        }else{ //Current char is outside current pair chars
            
            //Before resetting current pair, check if it deserves to be stored in longest pair
            if(currentPair.second>longestPairYet.second) longestPairYet=currentPair;
            
            //Reset current pair
            currentPair.first.first=s[i-1];
            currentPair.first.second=s[i];
            currentPair.second=lastChar.second+1;
        }
        
        //Maintain last character
        if(s[i]==lastChar.first) lastChar.second++;
        else{
            lastChar.first=s[i];
            lastChar.second=1;
        }
    }
    
    return longestPairYet;
}