main2.cpp

/// Source : https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/description/
/// Author : liuyubobobo
/// Time   : 2017-03-01
#include <iostream>
#include <vector>
#include <unordered_map>
#include <cassert>
#include <cmath>

using namespace std;


/// Two Pointers + Static Array as HashMap
/// Using self-built hashtable to record how many different characters in the current string
///
/// Time Complexity:  O(n)
/// Space Complexity: O(1)
class Solution {
public:
    int lengthOfLongestSubstringTwoDistinct(string s) {

        int hashtable[256] = {0};
        int l = 0, r = 0;
        int res = 0;
        int count = 0;
        while(r <= s.size()){
            if(r == s.size() || (count == 2 && !hashtable[s[r]])){
                res = max(res, r-l);
                while(count >= 2){
                    hashtable[s[l]] --;
                    if(hashtable[s[l]] == 0)
                        count --;
                    l ++;
                }

                if(r == s.size())
                    break;
            }
            else{
                hashtable[s[r]] ++;
                if(hashtable[s[r]] == 1)
                    count ++;
                r ++;
            }
        }

        return res;
    }
};

int main() {

    cout << Solution().lengthOfLongestSubstringTwoDistinct("eceba") << endl;
    cout << Solution().lengthOfLongestSubstringTwoDistinct("a") << endl;

    return 0;
}