longest consecutive elements sequence

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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.

 

使用map模板,

map记录了一个个的最大连续区间。并用两个关键字来检索,由小的检索到大的,和由大的检索到小的

主要思想是,新增的元素在两个区间之间既[..., a-1] a [a+1, ...] 合并区间。

为什么能得到最优解:贪心算法

因为每一个新元素,都尽可能的合并,取包含该元素的最大的连续区间。

 

 

using namespace std;class Solution {public:    int longestConsecutive(vector<int> &num) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        unordered_map<int, bool> set;        unordered_map<int, int> range;        int max = 0;        for(int i = 0; i < num.size(); i++)        {            if(!set[num[i]])            {                set[num[i]] = 1;                int left = num[i];                int right = num[i];                if(range.count(num[i] - 1) &&                     range[num[i] - 1] <= num[i] - 1)                    {                        left = range[num[i] - 1];                        range.erase(num[i] - 1);                    }                 if(range.count(num[i] + 1) &&                     range[num[i] + 1] >= num[i] + 1)                    {                        right = range[num[i] + 1];                        range.erase(num[i] + 1);                    }                 range[left] = right;                 range[right] = left;                 if(max < right - left + 1)                    max = right - left + 1;            }        }        return max;    }};


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