longest-palindromic-subsequence

array/longest-palindromic-subsequence.js

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+/**
+516. Longest Palindromic Subsequence
+
+Given a string s, find the longest palindromic subsequence's length in s.
+
+A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.
+
+Example 1:
+Input: s = "bbbab"
+Output: 4
+Explanation: One possible longest palindromic subsequence is "bbbb".
+
+Example 2:
+Input: s = "cbbd"
+Output: 2
+Explanation: One possible longest palindromic subsequence is "bb".
+
+Constraints:
+1 <= s.length <= 1000
+s consists only of lowercase English letters.
+*/
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var longestPalindromeSubseq = function (s) {
+  const length = s.length;
+  const memo = {};
+
+  const lps = (left, right) => {
+    const key = `${left},${right}`;
+
+    if (memo[key]) {
+      return memo[key];
+    }
+
+    if (left > right) {
+      return 0;
+    }
+
+    if (left === right) {
+      return 1;
+    }
+
+    if (s[left] === s[right]) {
+      memo[key] = lps(left + 1, right - 1) + 2;
+    } else {
+      memo[key] = Math.max(lps(left, right - 1), lps(left + 1, right));
+    }
+
+    return memo[key];
+  };
+
+  return lps(0, length - 1);
+};