題目鏈接

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.

分析：

算法1：首先想到的是排序，排序后遍歷一遍就可以找出最長連續序列的長度，只是要稍微注意下判斷連續序列的過程中有可能兩個元素相同，比如1 2 2 3，排序復雜度n*log(n)，雖然題目要求O(n)復雜度，但是這個解法也可以通過OJ，代碼如下：

1 class Solution { 2 public: 3 int longestConsecutive(vector<int> &num) { 4 // IMPORTANT: Please reset any member data you declared, as 5 // the same Solution instance will be reused for each test case. 6 int res = 1, len = num.size(); 7 if(len == 0)return 0; 8 sort(num.begin(), num.end()); 9 int curr = 1; 10 for(int i = 1; i < len; i++) 11 { 12 if(num[i] - num[i-1] == 1) 13 { 14 curr++; 15 if(curr > res)res = curr; 16 } 17 else if(num[i] - num[i-1] == 0); 18 else 19 curr = 1; 20 } 21 return res; 22 } 23 }; View Code

算法2：想要O(n)的算法，我們只有以時間換空間，先把數組中所有元素映射到哈希表。然后以題目給出的數組為例：對于100，先向下查找99沒找到，然后向上查找101也沒找到，那么連續長度是1，從哈希表中刪除100；然后是4，向下查找找到3,2,1，向上沒有找到5，那么連續長度是4，從哈希表中刪除4,3,2,1。這樣對哈希表中已存在的某個元素向上和向下查找，直到哈希表為空。算法相當于遍歷了一遍數組，然后再遍歷了一遍哈希表，復雜的為O(n)。代碼如下： ? ? ? ? ? ? ? ? ? ? ? ? ? ?本文地址

1 class Solution { 2 public: 3 int longestConsecutive(vector<int> &num) { 4 // IMPORTANT: Please reset any member data you declared, as 5 // the same Solution instance will be reused for each test case. 6 int res = 1, len = num.size(); 7 if(len == 0)return 0; 8 unordered_set<int> hashtable; 9 for(int i = 0; i < len; i++) 10 hashtable.insert(num[i]); 11 while(hashtable.empty() == false) 12 { 13 int currlen = 1; 14 int curr = *(hashtable.begin()); 15 hashtable.erase(curr); 16 int tmp = curr-1; 17 while(hashtable.empty()==false && 18 hashtable.find(tmp) != hashtable.end()) 19 { 20 hashtable.erase(tmp); 21 currlen++; 22 tmp--; 23 } 24 tmp = curr+1; 25 while(hashtable.empty()==false && 26 hashtable.find(tmp) != hashtable.end()) 27 { 28 hashtable.erase(tmp); 29 currlen++; 30 tmp++; 31 } 32 if(res < currlen)res = currlen; 33 } 34 return res; 35 } 36 };

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轉載于:https://www.cnblogs.com/TenosDoIt/p/3422249.html

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