Add day20

Jongeun/Day20/131_PalindromePartitioning.cpp

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+class Solution
+{
+public:
+    vector<vector<string>> partition(string s)
+    {
+        vector<vector<string>> res;
+        vector<string> sub;
+        _partition(res, sub, s);
+        return res;
+    }
+
+    void _partition(vector<vector<string>> &res, vector<string> &sub, string s)
+    {
+        if (isPalindrom(s))
+        {
+            sub.push_back(s);
+            res.push_back(sub);
+            sub.pop_back();
+        }
+
+        for (int i = 0; i < s.size() - 1; i++)
+        {
+            string left = s.substr(0, i + 1);
+            if (isPalindrom(left))
+            {
+                sub.push_back(left);
+                _partition(res, sub, s.substr(i + 1));
+                sub.pop_back();
+            }
+        }
+    }
+
+    bool isPalindrom(string &s)
+    {
+        int l = 0;
+        int r = s.size() - 1;
+        while (l <= r)
+        {
+            if (s[l] == s[r])
+            {
+                l++;
+                r--;
+            }
+            else
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+};

Jongeun/Day20/17_LetterCombinationsofaPhoneNumber.cpp

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+class Solution
+{
+public:
+    vector<string> letterCombinations(string digits)
+    {
+
+        // construct the map
+        unordered_map<char, string> m;
+        m['2'] = "abc";
+        m['3'] = "def";
+        m['4'] = "ghi";
+        m['5'] = "jkl";
+        m['6'] = "mno";
+        m['7'] = "pqrs";
+        m['8'] = "tuv";
+        m['9'] = "wxyz";
+
+        vector<string> res;
+        if (digits == "")
+        {
+            return res;
+        }
+
+        _letterCombinations(res, digits, m, "", 0);
+        return res;
+    }
+
+    void _letterCombinations(vector<string> &res, string &digits, unordered_map<char, string> &m, string comb, int start)
+    {
+        if (start == digits.size())
+        {
+            res.push_back(comb);
+            return;
+        }
+
+        for (auto c : m[digits[start]])
+        {
+
+            _letterCombinations(res, digits, m, comb + c, start + 1);
+        }
+    }
+};

Jongeun/Day20/355_DesignTwitter.cpp

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+class Twitter
+{
+public:
+    Twitter()
+    {
+    }
+
+    void postTweet(int userId, int tweetId)
+    {
+        tweets.push_back({userId, tweetId});
+    }
+
+    vector<int> getNewsFeed(int userId)
+    {
+        vector<int> res;
+        int num = 10;
+        unordered_set<int> temp = followMap[userId];
+        for (int i = tweets.size() - 1; i >= 0; i--)
+        {
+            if (tweets[i].first == userId || temp.find(tweets[i].first) != temp.end())
+            {
+                res.push_back(tweets[i].second);
+                num--;
+            }
+            if (num == 0)
+            {
+                break;
+            }
+        }
+
+        return res;
+    }
+
+    void follow(int followerId, int followeeId)
+    {
+        followMap[followerId].insert(followeeId);
+    }
+
+    void unfollow(int followerId, int followeeId)
+    {
+        followMap[followerId].erase(followeeId);
+    }
+
+private:
+    unordered_map<int, unordered_set<int>> followMap;
+    vector<pair<int, int>> tweets;
+};
+
+/**
+ * Your Twitter object will be instantiated and called as such:
+ * Twitter* obj = new Twitter();
+ * obj->postTweet(userId,tweetId);
+ * vector<int> param_2 = obj->getNewsFeed(userId);
+ * obj->follow(followerId,followeeId);
+ * obj->unfollow(followerId,followeeId);
+ */