[LeetCode]Longest Consecutive Sequence

Longest Consecutive Sequence

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.

hash table查找的时间复杂度O(1)，可以是时间复杂度降为O(n)。

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        if(nums.size()<1) return 0;
        int result=0;
        unordered_map<int,int> showed;
        for(int i=0;i<nums.size();i++)
        {
            showed[nums[i]]=1;
        }
        for(int i=0;i<nums.size();i++)
        {
            int number = nums[i];
            int times=0;
            if(showed.find(number)!=showed.end())
            {
                showed.erase(number);
                times=1;
                for(int j=number-1;;j--)
                {
                    if(showed.find(j)!=showed.end())
                    {
                        showed.erase(j);
                        times++;
                    }
                    else
                    {
                        break;
                    }
                }
                for(int j=number+1;;j++)
                {
                    if(showed.find(j)!=showed.end())
                    {
                        showed.erase(j);
                        times++;
                    }
                    else
                    {
                        break;
                    }
                }
            }
            result = max(result,times);
        }
        return result;
    }
};