// 873. Length of Longest Fibonacci Subsequence #include "leetcode.h" /* * a sequence 'x1, x2, ..., xn' is fibonacci like if 'n >= 3' and 'x_i + x_i + 1 * == x_i + 2' for all 'i + 2 <= n'. given a strictly increasing '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. */ static int a_max; int helper(int *arr, int *map, int i, int j, int sum) { int cnt = 0; while (sum <= a_max) { if (!map[sum]) break; cnt++; i = j; j = map[sum] - 1; sum = arr[i] + arr[j]; } return cnt; } int lenLongestFibSubseq(int *arr, int arrSize) { a_max = arr[arrSize - 1]; int map_size = a_max + 1; int *map = (int *)calloc(map_size, sizeof(int)); int cnt = 0, i, j, sum, tmp; for (int i = 0; i < arrSize; i++) map[arr[i]] = i + 1; for (int i = 0; i < arrSize; i++) { for (int j = i + 1; j < arrSize; j++) { sum = arr[i] + arr[j]; if (sum > a_max) break; if (!map[sum]) continue; tmp = helper(arr, map, i, j, sum); cnt = fmax(cnt, tmp); } } cnt += 2; free(map); return cnt > 2 ? cnt : 0; } int main() { int a1[] = {1, 2, 3, 4, 5, 6, 7, 8}, a2[] = {1, 3, 7, 11, 12, 14, 18}; printf("%d\n", lenLongestFibSubseq(a1, ARRAY_SIZE(a1))); // expect: 5 printf("%d\n", lenLongestFibSubseq(a2, ARRAY_SIZE(a2))); // expect: 3 }