add Longest Palindromic Substring solution

Author: HoonDongKang

longest-palindromic-substring/HoonDongKang.ts

@@ -0,0 +1,31 @@
+/**
+ * [Problem]: [5] Longest Palindromic Substring
+ * (https://leetcode.com/problems/longest-palindromic-substring/)
+ */
+// 시간복잡도 O(n^2)
+// 공간복잡도 O(1)
+function longestPalindrome(s: string): string {
+    if (s.length < 2) return s;
+
+    let start = 0;
+    let end = 0;
+
+    function findPalindrome(left: number, right: number): void {
+        while (0 <= left && right < s.length && s[left] === s[right]) {
+            left--;
+            right++;
+        }
+
+        if (right - left - 1 > end - start) {
+            start = left + 1;
+            end = right - 1;
+        }
+    }
+
+    for (let i = 0; i < s.length; i++) {
+        findPalindrome(i, i);
+        findPalindrome(i, i + 1);
+    }
+
+    return s.substring(start, end + 1);
+}