Add solution #873

Author: JoshCrozier

README.md

@@ -448,6 +448,7 @@
 867|[Transpose Matrix](./0867-transpose-matrix.js)|Easy|
 868|[Binary Gap](./0868-binary-gap.js)|Easy|
 872|[Leaf-Similar Trees](./0872-leaf-similar-trees.js)|Easy|
+873|[Length of Longest Fibonacci Subsequence](./0873-length-of-longest-fibonacci-subsequence.js)|Medium|
 875|[Koko Eating Bananas](./0875-koko-eating-bananas.js)|Medium|
 876|[Middle of the Linked List](./0876-middle-of-the-linked-list.js)|Easy|
 884|[Uncommon Words from Two Sentences](./0884-uncommon-words-from-two-sentences.js)|Easy|

solutions/0873-length-of-longest-fibonacci-subsequence.js

@@ -0,0 +1,42 @@
+/**
+ * 873. Length of Longest Fibonacci Subsequence
+ * https://leetcode.com/problems/length-of-longest-fibonacci-subsequence/
+ * Difficulty: Medium
+ *
+ * A sequence x1, x2, ..., xn is Fibonacci-like if:
+ * - n >= 3
+ * - xi + xi+1 == xi+2 for all i + 2 <= n
+ *
+ * Given a strictly increasing array arr of positive integers forming a sequence, return the length
+ * of the longest Fibonacci-like subsequence of arr. If one does not exist, return 0.
+ *
+ * A subsequence is derived from another sequence arr by deleting any number of elements (including
+ * none) from arr, without changing the order of the remaining elements. For example, [3, 5, 8] is
+ * a subsequence of [3, 4, 5, 6, 7, 8].
+ */
+
+/**
+ * @param {number[]} arr
+ * @return {number}
+ */
+var lenLongestFibSubseq = function(arr) {
+  const set = new Set(arr);
+  let max = 0;
+
+  for (let i = 0; i < arr.length; i++) {
+    for (let j = i + 1; j < arr.length; j++) {
+      let [x, y, count] = [arr[i], arr[j], 2];
+
+      while (set.has(x + y)) {
+        const next = x + y;
+        x = y;
+        y = next;
+        count++;
+      }
+
+      max = Math.max(max, count);
+    }
+  }
+
+  return max >= 3 ? max : 0;
+};