[LeetCode-Algorithms-128] "Longest Consecutive Sequence" (2017.12.14-WEEK15)
2017-12-14 13:42
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题目链接:Longest Consecutive Sequence
题目描述: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.
(1)思路:因为O(n)复杂度限制,因此不能将数组排序。使用一个集合存所有的数字,遍历数组中每个数字,如果其在集合中存在,那么将其移除,分别用两个变量pre和next表示它的前一个数和后一个数,如果pre在集合中,那么将pre移除,然后pre自减1,直至pre不在集合之中;next采用同样的方法,直至next和pre都不在集合中,则next-pre-1就是当前数字的最长连续序列。再用res存储这个序列的长度。遍历完数组中的所有元素后,所得到的最大的res即为最长序列长度。
(2)代码:
class Solution { public: int longestConsecutive(vector<int>& nums){ int res = 0; int length = nums.size(); set<int> s(nums.begin(), nums.end()); for(int i=0;i<length;i++) { if (!s.count(nums[i])) continue; s.erase(nums[i]); int pre = nums[i] - 1, next = nums[i] + 1; while (s.count(pre)) s.erase(pre--); while (s.count(next)) s.erase(next++); res = max(res, next - pre - 1); } return res; } };
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