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Problem
A sequence X_1, X_2, …, X_n is fibonacci-like if:
n >= 3X_i + X_{i+1} = X_{i+2} for all i + 2 <= nGiven a strictly increasing array A of positive integers forming a sequence, find the length of the longest fibonacci-like subsequence of A. If one does not exist, return 0.
(Recall that a subsequence is derived from another sequence A by deleting any number of elements (including none) from A, without changing the order of the remaining elements. For example, [3, 5, 8] is a subsequence of [3, 4, 5, 6, 7, 8].)
Example 1:
Input: [1,2,3,4,5,6,7,8]Output: 5Explanation:The longest subsequence that is fibonacci-like: [1,2,3,5,8].Example 2:
Input: [1,3,7,11,12,14,18]Output: 3Explanation:The longest subsequence that is fibonacci-like:[1,11,12], [3,11,14] or [7,11,18].
Note:
3 <= A.length <= 10001 <= A[0] < A[1] < … < A[A.length – 1] <= 10^9(The time limit has been reduced by 50% for submissions in Java, C, and C++.)
Solution
class Solution {
public int lenLongestFibSubseq(int[] A) {
Set<Integer> set = new HashSet<>();
for (int a: A) set.add(a);
int max = 2;
for (int i = 0; i < A.length-1; i++) {
for (int j = i+1; j < A.length; j++) {
int a1 = A[i], a2 = A[j];
int curMax = 2;
while (set.contains(a1+a2)) {
curMax++;
int temp = a1;
a1 = a2;
a2 = temp+a2;
}
max = Math.max(max, curMax);
}
}
return max == 2 ? 0 : max;
}
}