Leetcode 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.

代码如下：

class Solution {public:    int longestConsecutive(vector<int> &num) {        map<int,int> hashMap;        int result = 0;        for(int i=0;i<num.size();i++)        {            if(hashMap.find(num[i]) == hashMap.end())            {                int left=0,right = 0;                if(hashMap.find(num[i]-1) == hashMap.end())                    left = 0;                else                    left = hashMap[num[i]-1];                                if(hashMap.find(num[i]+1) == hashMap.end())                    right = 0;                else                    right = hashMap[num[i]+1];                                hashMap[num[i]] = right+left+1;                result = max(result,right+left+1);                                hashMap[num[i]-left] = right+left+1;                hashMap[num[i]+right] = left+right+1;            }            else                continue;        }        return result;    }};

对数据结构进行修改，代码如下：

class Solution {public:    int longestConsecutive(vector<int> &num) {        unordered_map<int,int> hashMap;        int result = 0;        for(int i=0;i<num.size();i++)        {            if(hashMap.find(num[i]) == hashMap.end())            {                int left=0,right = 0;                if(hashMap.find(num[i]-1) == hashMap.end())                    left = 0;                else                    left = hashMap[num[i]-1];                                if(hashMap.find(num[i]+1) == hashMap.end())                    right = 0;                else                    right = hashMap[num[i]+1];                                hashMap[num[i]] = right+left+1;                result = max(result,right+left+1);                                hashMap[num[i]-left] = right+left+1;                hashMap[num[i]+right] = left+right+1;            }            else                continue;        }        return result;    }};

另一种方法，就是先排序然后求连续子序列即可，代码如下：

class Solution {public:    int longestConsecutive(vector<int>& nums) {        if (nums.size() == 0) {            return 0;        }        sort(nums.begin(), nums.end());        int len = 0;        int temp = 1;        for (int i = 1; i < nums.size(); i++) {            if (nums[i] == nums[i - 1]) continue;            if (nums[i] == nums[i - 1] + 1) {                temp++;            } else {                len = max(len, temp);                temp = 1;            }        }        len = max(len, temp);        return len;    }};