内容简介:A sequenceGiven a
题目描述:
LeetCode 873. Length of Longest Fibonacci Subsequence
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:
<strong>Input: </strong>[1,2,3,4,5,6,7,8] <strong>Output: </strong>5 <strong>Explanation: </strong>The longest subsequence that is fibonacci-like: [1,2,3,5,8].
Example 2:
<strong>Input: </strong>[1,3,7,11,12,14,18] <strong>Output: </strong>3 <strong>Explanation</strong>: The longest subsequence that is fibonacci-like: [1,11,12], [3,11,14] or [7,11,18].
Note:
-
3 <= A.length <= 1000
-
1 <= 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++.)
题目大意:
求递增正整数组的最长类斐波那契数列。
解题思路:
动态规划(Dynamic Programming)
时间复杂度 O(N^2)
状态转移方程:
dp[y][x + y] = max(dp[y][x + y], dp[x][y] + 1)
上式中, dp[x][y]表示以(x, y)为结尾的类斐波那契数列的长度
Python代码:
class Solution(object): def lenLongestFibSubseq(self, A): """ :type A: List[int] :rtype: int """ vset = set(A) dp = collections.defaultdict(lambda: collections.defaultdict(int)) size = len(A) ans = 0 for i in range(size): x = A[i] for j in range(i + 1, size): y = A[j] if x + y not in vset: continue dp[y][x + y] = max(dp[y][x + y], dp[x][y] + 1) ans = max(dp[y][x + y], ans) return ans and ans + 2 or 0
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