LeetCode—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.

最好想到的思路是排序，如果排序，最快也要O（NlogN），不符合要求。

以空间换时间：

代码：

int longestConsecutive(vector<int>& nums)
{
    map<int,int> map1;
    int len = 0;
    int maxlen = len;
    for(int i = 0;i<nums.size();i++)
    {
       //在nums中出现，则不为0，否则为0
       map1[nums[i]]++;
    }
    int min1 = nums[0];
    int min_up = min1;
    int min_down = min1-1;
    int i = 0;
    while(i<nums.size())
    {
          if(map1[min_up]!=0)
          {
                len++;
                map1[min_up] = 0;
                min_up++;
          }
          if(map1[min_down]!=0)
          {
                 len++;
                 map1[min_down] = 0;
                 min_down--;
          }
          if(map1[min_down]==0 && map1[min_up]==0)
          {
                 if(maxlen<len)
                 maxlen = len;
                 cout<<maxlen<<endl;
                 len = 0;
                 while(map1[nums[i++]]==0&&i<nums.size());
                 if(i<nums.size())
                 {
                      min1 = nums[i-1];
                      min_up = min1;
                      min_down = min1-1;
                 }
                 else break;
          }
    }
    return maxlen;
}

时间复杂度：O(N),同时引入了map，空间复杂度O(N).