diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/f-exuan21.java b/construct-binary-tree-from-preorder-and-inorder-traversal/f-exuan21.java
new file mode 100644
index 000000000..12be66629
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/f-exuan21.java
@@ -0,0 +1,50 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+
+ // time : O(n) 
+ // space : O(n)
+ // n은 트리 노드 수
+
+ class Solution {
+
+    private int i = 0;
+    Map<Integer, Integer> map = new HashMap<>();
+
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+
+        for(int i = 0; i < inorder.length; i++) {
+            map.put(inorder[i], i);
+        }
+        
+        return build(preorder, inorder, 0, inorder.length);
+
+    }
+
+    private TreeNode build(int[] preorder, int[] inorder, int start, int end) {
+        if(i >= preorder.length || start >= end) {
+            return null;
+        }
+
+        int value = preorder[i++];
+        int index = map.get(value);
+
+        TreeNode leftTreeNode = build(preorder, inorder, start, index);
+        TreeNode rightTreeNode = build(preorder, inorder, index+1, end);
+
+        return new TreeNode(value, leftTreeNode, rightTreeNode);
+    }
+
+}
diff --git a/counting-bits/f-exuan21.java b/counting-bits/f-exuan21.java
new file mode 100644
index 000000000..35ecac809
--- /dev/null
+++ b/counting-bits/f-exuan21.java
@@ -0,0 +1,18 @@
+//time : O(n)
+//space : O(n)
+
+class Solution {
+
+    // f(n) = f(n >> 1) + (n & 1)
+
+    public int[] countBits(int n) {
+        int[] answer = new int[n + 1];
+
+        answer[0] = 0;
+        for(int i = 1; i <= n; i++) {
+            answer[i] = answer[i >> 1] + (i & 1);
+        }
+
+        return answer;
+    }
+}
diff --git a/valid-anagram/f-euxan21.java b/valid-anagram/f-euxan21.java
new file mode 100644
index 000000000..0e9a90493
--- /dev/null
+++ b/valid-anagram/f-euxan21.java
@@ -0,0 +1,24 @@
+// time : O(n)
+// space : O(1)
+
+class Solution {
+    public boolean isAnagram(String s, String t) {
+
+        if(s.length() != t.length()) return false;
+        
+        int[] charCount = new int[26];
+
+        for(int i = 0; i < s.length(); i++) {
+            charCount[s.charAt(i) - 'a']++;
+            charCount[t.charAt(i) - 'a']--;
+        }
+
+        for(int i : charCount) {
+            if(i != 0) {
+                return false;
+            }
+        }
+        
+        return true;
+    }
+}