【LeetCode】128. Longest Consecutive Sequence

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Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(n) complexity.

Example:

Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

题解：哈希表的利用，本来直接用hash数组但是有负数，这里用set保存，然后遍历num然后用cur记录当前数字所在连续数列的长度，ans表示最长长度，每次遍历num中的数x，如果x不在set中终止当前循环，然后while循环遍历x连续序列通过set判断是否在数组中，并把访问的数从set中删除，结束后cur重新设置为1，ans更新最大

代码如下：

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_set<int> record(num.begin(),num.end());
        if(num.size()==0)return 0;
        int cur=1,ans=1;
        for(int x:num){
            if(record.find(x)==record.end()) continue;
            record.erase(x);
            int prev=x-1;
            int next=x+1;
            while(record.find(prev)!=record.end()){
                cur++;record.erase(prev--);
            }
            while(record.find(next)!=record.end()){
                cur++;record.erase(next++);
            }
            
            ans=max(cur,ans);
            cur=1;
        }
        return ans;
    }
};