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[LeetCode] Longest Consecutive Sequence
Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
Your algorithm should run in O(n) complexity.
First of all,if there is no time complexity,we could sort the array,and then scan the array.Unfortunately,the time complexity is O(n*log n).It‘s been many times,when I am asked to solve a problem in O(n) which can be solved in O(n*log n) by using classical quicksort I would use a alternative method--hash table to do it.Here is the code.
1 class Solution { 2 public: 3 int longestConsecutive(vector<int> &num) { 4 unordered_map<int,bool> hashmap; 5 int maxlength = 0; 6 int curlength = 0; 7 8 for(vector<int>::iterator iter = num.begin(); 9 iter!=num.end();++iter){10 hashmap.insert(pair<int,bool>(*iter,true));11 }12 13 for(vector<int>::iterator iter = num.begin();14 iter!=num.end();++iter){15 int num = *iter;16 curlength = 0;17 while(hashmap.find(num) != hashmap.end()){18 ++curlength;19 hashmap.erase(num);20 ++num;21 }22 23 num = *iter-1;24 while(hashmap.find(num) != hashmap.end()){25 ++curlength;26 hashmap.erase(num);27 --num;28 }29 30 maxlength = maxlength<curlength?curlength:maxlength;31 }32 return maxlength;33 }34 };
[LeetCode] Longest Consecutive Sequence
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