Add palindromic-substrings solution in TS

Author: Jeehay28

palindromic-substrings/Jeehay28.ts

@@ -0,0 +1,97 @@
+// TC: O(n^2)
+// SC: O(1)
+function countSubstrings(s: string): number {
+  // Treat each index as the center of potential palindromic substrings
+  let cnt = 0;
+
+  for (let i = 0; i < s.length; i++) {
+    let start = i;
+    let end = i;
+
+    // Check for odd-length palindromes centered at i
+    while (start >= 0 && end < s.length && s[start] === s[end]) {
+      cnt++;
+      start--; // <----
+      end++; // ----->
+    }
+
+    // Check for even-length palindromes centered between i and i + 1
+    start = i;
+    end = i + 1;
+
+    while (start >= 0 && end < s.length && s[start] === s[end]) {
+      cnt++;
+      start--;
+      end++;
+    }
+  }
+
+  return cnt;
+}
+
+// TC: O(n^2)
+// SC: O(n^2)
+/*
+function countSubstrings(s: string): number {
+  // dp[(start, end)] = s[start] === s[end] && dp[(start + 1, end - 1)]
+
+  // A new starting character + an existing palindromic substring + a new ending character
+  //        start                 dp[(start + 1, end - 1)]                 end
+
+  const dp = new Map<string, boolean>();
+
+  for (let end = 0; end < s.length; end++) {
+    // start ranges from end down to 0
+    for (let start = end; start >= 0; start--) {
+      const key = `${start}-${end}`;
+      if (start === end) {
+        // "a"
+        dp.set(key, true);
+      } else if (start + 1 === end) {
+        // "aa"
+        dp.set(key, s[start] === s[end]);
+      } else {
+        const prevKey = `${start + 1}-${end - 1}`;
+        dp.set(key, s[start] === s[end] && dp.get(prevKey) === true);
+      }
+    }
+  }
+
+  let cnt = 0;
+
+  for (const [key, value] of dp.entries()) {
+    if (value) {
+      cnt++;
+    }
+  }
+
+  return cnt;
+}
+*/
+
+// TC: O(n^3)
+// SC: O(1)
+// function countSubstrings(s: string): number {
+//   const isPalindromic = (left: number, right: number) => {
+//     while (left < right) {
+//       if (s[left] !== s[right]) return false;
+
+//       left++;
+//       right--;
+//     }
+
+//     return true;
+//   };
+
+//   let cnt = 0;
+
+//   for (let i = 0; i < s.length; i++) {
+//     for (let j = i; j < s.length; j++) {
+//       if (isPalindromic(i, j)) {
+//         cnt++;
+//       }
+//     }
+//   }
+
+//   return cnt;
+// }