873. Length of Longest Fibonacci Subsequence
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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 ofA
. 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) fromA
, 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].
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++.)
Approach #1: unordered_map. [C++]
class Solution { public: int lenLongestFibSubseq(vector<int>& A) { unordered_map<int, int> memo; int len = A.size(); int ans = 0, temp = 0; for (int i = 0; i < len; ++i) memo[A[i]] = i; for (int i = 0; i < len; ++i) { for (int j = i + 1; j < len; ++j) { int ant = 2; int last_idx = i; for (int cur_idx = j; cur_idx < len; ) { temp = A[last_idx] + A[cur_idx]; if (memo.count(temp)) { ant++; last_idx = cur_idx; cur_idx = memo[temp]; } else break; } ans = max(ans, ant); } } return ans == 2 ? 0 : ans; } };
Approach #2: DP. [Java]
class Solution { public int lenLongestFibSubseq(int[] A) { int n = A.length; int res = 0; int[][] dp = new int[n+1][n+1]; for (int[] row : dp) Arrays.fill(row, 2); Map<Integer, Integer> pos = new HashMap<>(); for (int i = 0; i < n; ++i) pos.put(A[i], i); for (int j = 2; j < n; ++j) { for (int i = j-1; i > 0; --i) { int prev = A[j] - A[i]; if (prev >= A[i]) break; if (!pos.containsKey(prev)) continue; dp[i][j] = dp[pos.get(prev)][i] + 1; res = Math.max(res, dp[i][j]); } } return res; } }
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