feat: add Palindromic Substrings

Author: dukeluo

README.md

@@ -84,6 +84,7 @@ Roadmap: https://neetcode.io/roadmap
 | 198  | [House Robber](https://leetcode.com/problems/house-robber/description/)                                                                                           | Medium     | [ts](./TypeScript/70.climbing-stairs.ts)                                                      | 1-D DP           |
 | 213  | [House Robber II](https://leetcode.com/problems/house-robber-ii/description/)                                                                                     | Medium     | [ts](./TypeScript/213.house-robber-ii.ts)                                                     | 1-D DP           |
 | 5    | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/description/)                                                         | Medium     | [ts](./TypeScript/5.longest-palindromic-substring.ts)                                         | 1-D DP           |
+| 647  | [Palindromic Substrings](https://leetcode.com/problems/palindromic-substrings/description/)                                                                       | Medium     | [ts](./TypeScript/647.palindromic-substrings.ts)                                              | 1-D DP           |
 
 ### Others
 

TypeScript/647.palindromic-substrings.ts

@@ -0,0 +1,28 @@
+// dp[i+1][j-1] === true && s[i] === s[j] => dp[i][j] === true
+function countSubstrings(s: string): number {
+  let dp: boolean[][] = [];
+  let count = 0;
+
+  for (let i = 0; i < s.length; i++) {
+    dp[i] = [];
+    dp[i][i] = true;
+    count = count + 1;
+  }
+
+  for (let i = s.length - 1; i >= 0; i--) {
+    for (let j = i + 1; j < s.length; j++) {
+      if (j - i === 1 || dp[i + 1][j - 1]) {
+        dp[i][j] = s[i] === s[j];
+      }
+
+      if (dp[i][j]) {
+        count = count + 1;
+      }
+    }
+  }
+
+  return count;
+}
+
+console.log(countSubstrings("abc"));
+console.log(countSubstrings("aaa"));