longest-consecutive-sequence

longest-consecutive-sequence

题目描述

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

For example,

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.

思路

题目要求线性时间内实现，先排序再查找这种方法被摒弃了

1. 用一个set集合存储数组

2. 对原数组遍历

   如果元素在集合中，对其左右两边搜索（比如3， 搜索是否2，1，0….在集合，搜索4，5，6,……是否在集合中，集合必须删除访问过的节点

3. 返回最大连续长度

代码

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_set<int> myset(num.begin(),num.end());
        int res=1;
        for(int current:num)
        {
            if(myset.find(current)==myset.end()) continue;

            myset.erase(current);
            int prev=current-1,post=current+1;
            while(myset.find(prev)!=myset.end())
            {
                myset.erase(prev--);
            }
            while(myset.find(post)!=myset.end())
            {
                myset.erase(post++);
            }
            res=max(res,post-prev-1);
        }

        return res;
    }
};