LeetCode OJ:Longest Consecutive Sequence

Longest Consecutive Sequence

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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.

O(nlogn)复杂度即先排序的实现如下，虽然AC，但不满足题目要求

class Solution{public:int longestConsecutive(vector<int> &num){sort(&num[0],&num[0]+num.size());int maxV=1;int count=1;for(int i=1;i<num.size();i++){    if(num[i]==num[i-1])continue;if(num[i]-num[i-1]==1){count++;}else{maxV=maxV<count?count:maxV;count=1;}}return maxV<count?count:maxV;}};

O(n)时间复杂度

class Solution {public:    int longestConsecutive(vector<int> &num) {        unordered_map<int,bool> vis;        for(auto i:num)vis[i]=false;                int longest=0;                for(auto i:num){            if(vis[i])continue;                        int length=1;                        vis[i]=true;                        for(int j=i+1;vis.find(j)!=vis.end();++j){                vis[j]=true;                ++length;            }            for(int j=i-1;vis.find(j)!=vis.end();--j){                vis[j]=true;                ++length;            }                        longest=max(longest,length);        }        return longest;    }};