### HARRYPOTTER0's blog

By HARRYPOTTER0, history, 9 months ago, ,

Given an int array which might contain duplicates, find the largest subset of it which form a sequence. Ex: {1,6,10,4,7,9,5} then ans is 4,5,6,7

Sorting is an obvious solution. Can this be done in O(n) time ??

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 » 9 months ago, # |   0 I have a simple idea, but I am not sure whether this is correct or not.We can adopt a "hash table" to record the integers that have appeared in the set, i.e., for some integer N, if it appears in the set, then hash[N] = 1; otherwise hash[N] = 0. Then, the problem is equivalent to finding out the longest sub-sequence consisting of "1"s. If I were correct, this solution has complexity O(n). However, this approach fails if the maximum integer turns out to be too large, for instance, 109.
•  » » 9 months ago, # ^ |   0 It's a good approach :) I have implemented it here......Solution
•  » » » 9 months ago, # ^ |   0 Your code fails on [1,2,3,5,6,7,4].
 » 9 months ago, # | ← Rev. 4 →   +15 On the one hand, using a hash table, we can dfs/bfs and find connected components on the path graph (add an edge for each adjacent elements) to solve the problem in Θ(n) expected time.On the other hand, on the algebraic decision tree model, your problem cannot be solved in O(n) time. To see this, take an instance of the set disjointness problem (A, B) and transform it to [3A1, ..., 3An, 3B1 + 1, ..., 3Bn + 1]. Because the set disjointness problem has lower bound [1], your problem has the same bound. [1] Ben-Or, Michael. Lower bounds for algebraic computation trees.
•  » » 9 months ago, # ^ |   0 Won't this work Here's a in-place algorithm with O(n) time and O(1) extra-space Given array 'A' of size 'n', the goal is to reorder elements in the given array so that they are in their correct positions i.e. A[i]-min(A) is at A[0] when A[i]==min(A) and A[j]-min(A) is at A[1] if A[j] = min(A)+1 and A[k]-min(A) is at A[2] if A[k] = min(A)+2 ..... so on.... Pass1: Find max(A) and min(A) if ( max(A)-min(A) > n ) then return false //i.e. you cannot have a sequence greater than n Pass2: For every element 'i', a. if A[i] == A[A[i]-min(A)] //already at the right position if i != arr[i]-minArr then set A[i]=-Inf //this is a duplicate else continue next iteration b. else //swap to move it to the right position swap A[i] with A[A[i]-min(A)] after swapping if A[i] != min(A)+i repeat from step 'a' Pass3: For every element 'i', check if next element == A[i]+1, if not then return false. An example with duplicates: {45,50,47,45,50,46,49,48,49} Pass1: max(A) = 50, min(A) = 45 Pass2: modified Array: [45,50,47,45,50,46,49,48,49] //45 already at A[A[0]-min(A)] [45,46,47,45,50,50,49,48,49] //swap 50 & 46 [45,46,47,45,50,50,49,48,49] //47 already at A[47-45] [45,46,47,-Inf,50,50,49,48,49] //A[3] = -Inf since it is a duplicate [45,46,47,-Inf,-Inf,50,49,48,49] [45,46,47,-Inf,-Inf,50,49,48,49] [45,46,47,-Inf,49,50,-Inf,48,49] [45,46,47,48,49,50,-Inf,-Inf,49] [45,46,47,48,49,50,-Inf,-Inf,-Inf] Pass3: return true//Note : instead of -Inf you can use some other marker such as min(A)-2