diff --git a/longest-palindromic-substring/gitsunmin.ts b/longest-palindromic-substring/gitsunmin.ts
new file mode 100644
index 000000000..535c01c1a
--- /dev/null
+++ b/longest-palindromic-substring/gitsunmin.ts
@@ -0,0 +1,45 @@
+/**
+ * https://leetcode.com/problems/longest-palindromic-substring/
+ * time complexity : O(n)
+ * space complexity : O(n^2)
+ */
+
+function findPalindromeAroundCenter(s: string, leftIndex: number, rightIndex: number): [number, number] {
+    while (leftIndex >= 0 && rightIndex < s.length && s[leftIndex] === s[rightIndex]) {
+        leftIndex--; // 왼쪽으로 확장
+        rightIndex++; // 오른쪽으로 확장
+    }
+
+    // 팰린드롬의 시작과 끝 인덱스 반환
+    return [leftIndex + 1, rightIndex - 1];
+}
+
+function longestPalindrome(s: string): string {
+    if (s.length <= 1) return s;
+
+    let longestPalindromeStartIndex = 0;
+    let longestPalindromeLength = 0;
+
+    for (let centerIndex = 0; centerIndex < s.length; centerIndex++) {
+        // 홀수 길이 팰린드롬 확인
+        const [oddStart, oddEnd] = findPalindromeAroundCenter(s, centerIndex, centerIndex);
+        const oddLength = oddEnd - oddStart + 1;
+
+        if (oddLength > longestPalindromeLength) {
+            longestPalindromeStartIndex = oddStart;
+            longestPalindromeLength = oddLength;
+        }
+
+        // 짝수 길이 팰린드롬 확인
+        const [evenStart, evenEnd] = findPalindromeAroundCenter(s, centerIndex, centerIndex + 1);
+        const evenLength = evenEnd - evenStart + 1;
+
+        if (evenLength > longestPalindromeLength) {
+            longestPalindromeStartIndex = evenStart;
+            longestPalindromeLength = evenLength;
+        }
+    }
+
+    // 가장 긴 팰린드롬 부분 문자열 반환
+    return s.substring(longestPalindromeStartIndex, longestPalindromeStartIndex + longestPalindromeLength);
+}
diff --git a/subtree-of-another-tree/gitsunmin.ts b/subtree-of-another-tree/gitsunmin.ts
new file mode 100644
index 000000000..21e156a2c
--- /dev/null
+++ b/subtree-of-another-tree/gitsunmin.ts
@@ -0,0 +1,29 @@
+/**
+ * https://leetcode.com/problems/subtree-of-another-tree/
+ * time complexity : O(n * m)
+ * space complexity : O(n)
+ */
+
+class TreeNode {
+    val: number;
+    left: TreeNode | null;
+    right: TreeNode | null;
+    constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+        this.val = val ?? 0;
+        this.left = left ?? null;
+        this.right = right ?? null;
+    }
+}
+
+function isSameTree(p: TreeNode | null, q: TreeNode | null): boolean {
+    if (!p && !q) return true;
+    if (!p || !q || p.val !== q.val) return false;
+
+    return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
+}
+
+function isSubtree(root: TreeNode | null, subRoot: TreeNode | null): boolean {
+    if (!root) return false;
+    if (isSameTree(root, subRoot)) return true;
+    return isSubtree(root.left, subRoot) || isSubtree(root.right, subRoot);
+}
diff --git a/validate-binary-search-tree/gitsunmin.ts b/validate-binary-search-tree/gitsunmin.ts
new file mode 100644
index 000000000..9d0c2c9e2
--- /dev/null
+++ b/validate-binary-search-tree/gitsunmin.ts
@@ -0,0 +1,32 @@
+/**
+ * https://leetcode.com/problems/validate-binary-search-tree/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+
+class TreeNode {
+    val: number;
+    left: TreeNode | null;
+    right: TreeNode | null;
+    constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+        this.val = val ?? 0;
+        this.left = left ?? null;
+        this.right = right ?? null;
+    }
+}
+
+function validate(node: TreeNode | null, lower: number, upper: number): boolean {
+    if (!node) return true;
+    const val = node.val;
+
+    if (val <= lower || val >= upper) return false;
+    if (!validate(node.right, val, upper)) return false;
+    if (!validate(node.left, lower, val)) return false;
+
+    return true;
+}
+
+function isValidBST(root: TreeNode | null): boolean {
+
+    return validate(root, -Infinity, Infinity);
+}