LeetCode 873. Length of Longest Fibonacci Subsequence
原题链接在这里:https://leetcode.com/problems/length-of-longest-fibonacci-subsequence/
题目:
A sequence X_1, X_2, ..., X_n
is fibonacci-like if:
n >= 3
X_i + X_{i+1} = X_{i+2}
for alli + 2 <= n
Given 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: 5
Explanation:
The longest subsequence that is fibonacci-like: [1,2,3,5,8].
Example 2:
Input: [1,3,7,11,12,14,18]
Output: 3
Explanation:
The longest subsequence that is fibonacci-like:
[1,11,12], [3,11,14] or [7,11,18].
题解:
Take a = A[i] and b = A[j] as first 2 elements, j>i.
Check if A contains A[i]+A[j]. If yes, then a = b, b = a+b, and fibonacci sequence length ++.
Until A doesn't contian (A[i] + A[j]), update the global longest length.
Time Complexity: O(n^2 * logM). n = A.length. M is the largest number in A, since in while loop it grows exponentially, while takes logM.
Space: O(n).
AC Java:
class Solution {
public int lenLongestFibSubseq(int[] A) {
if(A == null || A.length < 3){
return 0;
} HashSet<Integer> hs = new HashSet<>();
for(int a : A){
hs.add(a);
} int res = 0; int n = A.length;
for(int i = 0; i<n-2; i++){
for(int j = i+1; j<n-1; j++){
int a = A[i];
int b = A[j];
int l = 2;
while(hs.contains(a+b)){
int temp = a;
a = b;
b = temp+b;
l++;
} res = Math.max(res, l);
}
} return res > 2 ? res : 0;
}
}
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