Add week 15 solutions: longest-palindromic-substring/

Author: gitsunmin

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);
+}