diff --git a/YIM2UL2ET/README.md b/YIM2UL2ET/README.md
index d868578..57c0ce8 100644
--- a/YIM2UL2ET/README.md
+++ b/YIM2UL2ET/README.md
@@ -19,5 +19,5 @@
 | 15차시 | 2024.04.09 |  구현  | [BOJ 14719](https://www.acmicpc.net/problem/14719)  | [BOJ 14719 풀이](https://github.com/AlgoLeadMe/AlgoLeadMe-7/pull/48) |
 | 16차시 | 2024.04.11 |  애드 혹  | [BOJ 15927](https://www.acmicpc.net/problem/15927)  | [BOJ 15927 풀이](https://github.com/AlgoLeadMe/AlgoLeadMe-7/pull/51) |
 | 17차시 | 2024.04.15 |  DP  | [BOJ 11726](https://www.acmicpc.net/problem/11726)  | [BOJ 11726 풀이](https://github.com/AlgoLeadMe/AlgoLeadMe-7/pull/53) |
-
+| 18차시 | 2024.04.15 |  트리  | [BOJ 5639](https://www.acmicpc.net/problem/5639)  | [BOJ 5639 풀이](https://github.com/AlgoLeadMe/AlgoLeadMe-7/pull/53) |
 ---
\ No newline at end of file
diff --git "a/YIM2UL2ET/\355\212\270\353\246\254/18\354\260\250\354\213\234 - BOJ 5639.cpp" "b/YIM2UL2ET/\355\212\270\353\246\254/18\354\260\250\354\213\234 - BOJ 5639.cpp"
new file mode 100644
index 0000000..ed5a18c
--- /dev/null
+++ "b/YIM2UL2ET/\355\212\270\353\246\254/18\354\260\250\354\213\234 - BOJ 5639.cpp"	
@@ -0,0 +1,34 @@
+#include <iostream>
+using namespace std;
+
+struct node {
+    int value = -1;
+    node* right = NULL;
+    node* left = NULL;
+};
+
+void input(node &nd, int n) {
+    if (nd.value == -1) nd.value = n;
+    else if (nd.value > n) {
+        if (nd.left == NULL) nd.left = new node;
+        input(*nd.left, n); 
+    }
+    else {
+        if (nd.right == NULL) nd.right = new node;
+        input(*nd.right, n); 
+    }
+}
+
+void output(node &nd) {
+    if (nd.left != NULL) output(*nd.left);
+    if (nd.right != NULL) output(*nd.right);
+    cout << nd.value << '\n';
+}
+
+int main(void) {
+    int n;
+    node root;
+    while (cin >> n) input(root, n);
+    output(root);
+    return 0;
+}
\ No newline at end of file