Add solution #647

Author: JoshCrozier

README.md

@@ -486,6 +486,7 @@
 643|[Maximum Average Subarray I](./0643-maximum-average-subarray-i.js)|Easy|
 645|[Set Mismatch](./0645-set-mismatch.js)|Medium|
 646|[Maximum Length of Pair Chain](./0646-maximum-length-of-pair-chain.js)|Medium|
+647|[Palindromic Substrings](./0647-palindromic-substrings.js)|Medium|
 648|[Replace Words](./0648-replace-words.js)|Medium|
 649|[Dota2 Senate](./0649-dota2-senate.js)|Medium|
 653|[Two Sum IV - Input is a BST](./0653-two-sum-iv-input-is-a-bst.js)|Easy|

solutions/0647-palindromic-substrings.js

@@ -0,0 +1,36 @@
+/**
+ * 647. Palindromic Substrings
+ * https://leetcode.com/problems/palindromic-substrings/
+ * Difficulty: Medium
+ *
+ * Given a string s, return the number of palindromic substrings in it.
+ *
+ * A string is a palindrome when it reads the same backward as forward.
+ *
+ * A substring is a contiguous sequence of characters within the string.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var countSubstrings = function(s) {
+  let result = 0;
+
+  for (let i = 0; i < s.length; i++) {
+    result += expand(i, i);
+    result += expand(i, i + 1);
+  }
+
+  return result;
+
+  function expand(left, right) {
+    let count = 0;
+    while (left >= 0 && right < s.length && s[left] === s[right]) {
+      count++;
+      left--;
+      right++;
+    }
+    return count;
+  }
+};