5. Longest Palindromic Substring (axross#121)

Author: Kohei Asai

solutions/longest_palindromic_substring.ts

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+// 5. Longest Palindromic Substring
+// https://leetcode.com/problems/longest-palindromic-substring/
+export default function longestPalindrome(s: string): string {
+  let maxPalindromeFrom = 0;
+  let maxPalindromeTo = 0;
+
+  for (let i = 0; i < s.length; ++i) {
+    const maxRadius = Math.min(s.length - i - 1, i - 0);
+    let palindromicFrom = i;
+    let palindromicTo = i + 1;
+
+    for (let radius = 1; radius <= maxRadius; ++radius) {
+      if (s[i - radius] !== s[i + radius]) break;
+
+      palindromicFrom -= 1;
+      palindromicTo += 1;
+    }
+
+    if (palindromicTo - palindromicFrom > maxPalindromeTo - maxPalindromeFrom) {
+      maxPalindromeFrom = palindromicFrom;
+      maxPalindromeTo = palindromicTo;
+    }
+
+    if (s[i + 1] && s[i + 1] === s[i]) {
+      const maxRadius = Math.min(s.length - i - 2, i - 0);
+      let palindromicFrom = i;
+      let palindromicTo = i + 2;
+
+      for (let radius = 1; radius <= maxRadius; ++radius) {
+        if (s[i - radius] !== s[i + 1 + radius]) break;
+
+        palindromicFrom -= 1;
+        palindromicTo += 1;
+      }
+
+      if (
+        palindromicTo - palindromicFrom >
+        maxPalindromeTo - maxPalindromeFrom
+      ) {
+        maxPalindromeFrom = palindromicFrom;
+        maxPalindromeTo = palindromicTo;
+      }
+    }
+  }
+
+  return s.substring(maxPalindromeFrom, maxPalindromeTo);
+}

solutions/longest_palindromic_substring_test.ts

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+import { test } from "https://deno.land/std/testing/mod.ts";
+import { assert } from "https://deno.land/std/testing/asserts.ts";
+import longestPalindrome from "./longest_palindromic_substring.ts";
+
+test("5. Longest Palindromic Substring", () => {
+  assert(["bab", "aba"].includes(longestPalindrome("babad")));
+  assert(longestPalindrome("cbbd") === "bb");
+});