diff --git a/best-time-to-buy-and-sell-stock/YoungSeok-Choi.java b/best-time-to-buy-and-sell-stock/YoungSeok-Choi.java
new file mode 100644
index 000000000..2e70e2689
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/YoungSeok-Choi.java
@@ -0,0 +1,24 @@
+// NOTE: tc --> O(n)
+class Solution {
+    public int maxProfit(int[] prices) {
+
+        int curMax = 0;
+        int gMax = 0;
+
+        if(prices.length == 0) return 0;
+        
+        int sell = prices[0];
+        for(int i = 1; i < prices.length; i++) {
+            curMax = Math.max(0, prices[i] - sell);
+
+            // NOTE: 새롭게 시작하는게 더 좋은경우
+            if(curMax == 0) {
+                sell = prices[i];
+            }
+
+            gMax = Math.max(curMax, gMax); 
+        }
+
+        return gMax;        
+    }
+}
diff --git a/encode-and-decode-strings/YoungSeok-Choi.java b/encode-and-decode-strings/YoungSeok-Choi.java
new file mode 100644
index 000000000..6a1925602
--- /dev/null
+++ b/encode-and-decode-strings/YoungSeok-Choi.java
@@ -0,0 +1,45 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class Solution {
+    /*
+     * @param strs: a list of strings
+     * @return: encodes a list of strings to a single string.
+     */
+    public String encode(List<String> strs) {
+        List<String> temp = new ArrayList<>();
+
+        if(strs.size() == 0) return null;
+
+        for(String s : strs) {
+            if(":".equals(s)) {
+                temp.add("::");
+            } else {
+                temp.add(s);
+            }            
+        }
+
+        return String.join(":;", temp);
+    }
+
+    /*
+     * @param str: A string
+     * @return: decodes a single string to a list of strings
+     */
+    public List<String> decode(String str) {
+        List<String> temp = new ArrayList<>();
+
+        if(str == null) return new ArrayList<>();
+
+        // if(str.length() == 0) return new ArrayList<>();
+        
+        for(String s : str.split(":;")) {
+            if("::".equals(s)) {
+                temp.add(":");
+            } else {
+                temp.add(s);
+            }            
+        }
+        return temp;
+    }
+}
diff --git a/group-anagrams/YoungSeok-Choi.java b/group-anagrams/YoungSeok-Choi.java
new file mode 100644
index 000000000..8ede468fc
--- /dev/null
+++ b/group-anagrams/YoungSeok-Choi.java
@@ -0,0 +1,34 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+// NOTE: tc -> O(n)
+class Solution {
+    public List<List<String>> groupAnagrams(String[] strs) {
+
+        List<List<String>> result = new ArrayList<>();
+        Map<String, List<String>> sMap = new HashMap<>();
+
+        for(int i = 0; i < strs.length; i++) {
+            char[] cArr = strs[i].toCharArray();
+            Arrays.sort(cArr);
+            String sorted = new String(cArr);
+
+            if(sMap.containsKey(sorted)) {
+                sMap.get(sorted).add(strs[i]);
+            } else {
+                List<String> temp = new ArrayList<>();
+                temp.add(strs[i]);
+                sMap.put(sorted, temp);
+            }
+        }
+
+        for(List<String> arr : sMap.values()) {
+            result.add(arr);
+        }
+
+        return result;
+    }
+}
diff --git a/implement-trie-prefix-tree/YoungSeok-Choi.java b/implement-trie-prefix-tree/YoungSeok-Choi.java
new file mode 100644
index 000000000..54e2dfcc9
--- /dev/null
+++ b/implement-trie-prefix-tree/YoungSeok-Choi.java
@@ -0,0 +1,37 @@
+import java.util.HashMap;
+import java.util.Map;
+
+// Map으로 풀려버려서 당황..
+// 이진트리? 어떤식으로 풀어야 할지 자료구조 정하고 다시 풀어보기..
+class Trie {
+
+    Map<String, Boolean> tMap;
+
+    public Trie() {
+        this.tMap = new HashMap<>();
+    }
+    
+    public void insert(String word) {
+        this.tMap.put(word, true);
+    }
+    
+    public boolean search(String word) {
+        return this.tMap.containsKey(word);
+    }
+    
+    public boolean startsWith(String prefix) {
+        for(String key : this.tMap.keySet()) {
+            if(key.startsWith(prefix)) return true;
+        }
+
+        return false;
+    }
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie obj = new Trie();
+ * obj.insert(word);
+ * boolean param_2 = obj.search(word);
+ * boolean param_3 = obj.startsWith(prefix);
+ */
diff --git a/word-break/YoungSeok-Choi.java b/word-break/YoungSeok-Choi.java
new file mode 100644
index 000000000..22f98abe8
--- /dev/null
+++ b/word-break/YoungSeok-Choi.java
@@ -0,0 +1,80 @@
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+// DFS 완전탐색할 때 불필요한 탐색을 줄이는 방법을 항상 고민할 것.
+class Solution {
+    public int size = 0;
+    public Map<String, Boolean> failedMap = new HashMap<>();
+    public boolean wordBreak(String s, List<String> wordDict) {
+        size = wordDict.size();
+
+        Set<Character> sSet = new HashSet<>();
+        for (char c : s.toCharArray()) {
+            sSet.add(c);
+        }   
+
+        Set<Character> wSet = new HashSet<>();
+        for (char c : String.join("", wordDict).toCharArray()) {
+            wSet.add(c);
+        }
+
+        if(sSet.size() > wSet.size()) {
+            return false;
+        }
+
+        return dfs(s, wordDict);
+    }
+
+    public boolean dfs(String s, List<String> wordDict) {
+        if(s.length() == 0) return true;
+        if(failedMap.containsKey(s)) return false;
+
+        for(int i = 0; i < size; i++) {
+            String word = wordDict.get(i);
+            if(s.startsWith(word)) {
+
+                s = s.substring(word.length());
+                boolean result = dfs(s, wordDict);
+
+                if(result) {
+                    return true;
+                } else {
+                    failedMap.put(s, true);
+                }
+
+                s = word + s;                
+            }
+        }
+
+        return false;
+    }
+}
+
+// 특정 문자로 시작되는 것만 판단하여 반복해 풀려고 했던 접근법.
+// 전체 조합을 보아야 하는 "cars", ["cars", "ca", "rs"] 경우에 반례가 됨.
+class WrongSolution {
+    public boolean wordBreak(String s, List<String> wordDict) {
+        int size = wordDict.size();
+        
+        while(true) {
+            boolean isMatched = false;
+            for(int i = 0; i < size; i++) {
+                String word = wordDict.get(i);
+                if(s.startsWith(word)) {
+                    isMatched = true;
+                    s = s.substring(word.length());
+                    break;
+                }
+            }
+
+            if(!isMatched) {
+                break;
+            }
+        }
+
+        return s.length() == 0;   
+    }
+}