kt: add «873. Length of Longest Fibonacci Subsequence»

URL:	https://leetcode.com/problems/length-of-longest-fibonacci-subsequence/

kt/length-of-longest-fibonacci-subsequence.kt

@@ -0,0 +1,33 @@
+class Solution {
+    fun lenLongestFibSubseq(arr: IntArray): Int {
+        val n = arr.size
+
+        val dp = Array(n) { IntArray(n) }
+        var maxLen = 0
+
+        (2..n - 1).forEach {
+            var (l, r) = 0 to it - 1
+
+            while (l < r) {
+                val sum = arr[l] + arr[r]
+
+                when {
+                    sum > arr[it] -> r--
+                    sum < arr[it] -> l++
+                    else -> {
+                        dp[r][it] = dp[l][r] + 1
+                        maxLen = listOf(maxLen, dp[r][it]).max()!!
+
+                        r--
+                        l++
+                    }
+                }
+            }
+        }
+
+        return when (maxLen) {
+            0 -> 0
+            else -> maxLen + 2
+        }
+    }
+}