Palindromic Substrings

Author: Sunjae95

palindromic-substrings/sunjae95.js

@@ -0,0 +1,90 @@
+/**
+ * @description
+ * time complexity: O(N^3)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. stack, permutation
+ * 2. Brute force
+ *
+ * strategy:
+ * Brute force, calculate
+ *
+ * reason:
+ * intuitive way
+ *
+ * @param {string} s
+ * @return {number}
+ */
+var countSubstrings = function (s) {
+  let answer = 0;
+  const len = s.length;
+
+  for (let i = 0; i < len; i++) {
+    for (let j = i + 1; j < len + 1; j++) {
+      const subStr = s.slice(i, j);
+      if (isPalindrome(subStr)) answer++;
+    }
+  }
+
+  return answer;
+};
+
+function isPalindrome(str) {
+  const len = str.length;
+  const middleIndex = Math.floor(len / 2);
+
+  for (let i = 0; i < middleIndex; i++) {
+    if (str[i] !== str[len - 1 - i]) return false;
+  }
+
+  return true;
+}
+
+/**
+ * @description
+ * time complexity: O(N^2)
+ * space complexity: O(N^2)
+ *
+ * brainstorming:
+ * 1. https://sbslc.tistory.com/56
+ *
+ * strategy:
+ * dynamic programming
+ *
+ * reason:
+ * to challenge dp
+ *
+ * @param {string} s
+ * @return {number}
+ */
+var countSubstrings = function (s) {
+  const answer = [];
+  const MAX_LENGTH = s.length;
+  const dp = Array.from({ length: MAX_LENGTH }, (_, i) =>
+    Array.from({ length: MAX_LENGTH }, (_, j) => {
+      if (i === j) answer.push(s[i]);
+      return i === j;
+    })
+  );
+  // Check next step ex) aa, bb, cc
+  for (let i = 0; i < MAX_LENGTH - 1; i++) {
+    const nextIndex = i + 1;
+    if (s[i] === s[nextIndex]) {
+      dp[i][nextIndex] = true;
+      answer.push(s.slice(i, nextIndex + 1));
+    }
+  }
+  // Check against values calculated in the previous step
+  for (let len = 2; len <= MAX_LENGTH; len++) {
+    for (let i = 0; i < MAX_LENGTH - len; i++) {
+      const lastIndex = len + i;
+      if (s[i] === s[lastIndex] && dp[i + 1][lastIndex - 1]) {
+        dp[i][lastIndex] = true;
+        answer.push(s.slice(i, lastIndex + 1));
+      }
+    }
+  }
+
+  return answer.length;
+};