Create 873. Length of Longest Fibonacci Subsequence (#726)

Author: Chayandas07

873. Length of Longest Fibonacci Subsequence

@@ -0,0 +1,26 @@
+class Solution {
+public:
+    int lenLongestFibSubseq(vector<int>& arr) {
+        int  n = arr.size();
+        unordered_map<int,int>mp; // element->index
+        for(int i=0; i<n; i++){
+            mp[arr[i]] = i;
+        }
+        vector<vector<int>>dp(n,vector<int>(n,2));
+        int maxLen = 0;
+        
+        for(int i=0; i<n-1;i++){
+            for(int j=i+1; j<n;j++){
+                int next = arr[i] + arr[j];
+                if(mp.find(next) != mp.end()){
+                    int k = mp[next];
+                    if(k>j){
+                        dp[j][k] = 1 + dp[i][j];
+                        maxLen = max(maxLen,dp[j][k]);
+                    }
+                }
+            }
+        }  
+        return maxLen > 2 ? maxLen : 0;
+    }
+};