*leetcode #128 in cpp

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本文介绍了一种寻找未排序整数数组中最长连续元素序列的算法。通过使用映射存储数据，该算法能在O(n)的时间复杂度内找到最长连续序列的长度。示例中展示了一个具体的实现案例。

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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.

Solution:

We store the number into a map. For each number, we try to find its right and its left as far as we can, by check if map contains number + 1 and number - 1. If we cannot find any left or right, this means the consecutive list containing the number has terminated

Code:

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        map<int,int> mapp;
        for(int i =0; i < nums.size(); i ++){
            mapp[nums[i]] = 1; 
        }
        int max_len = 0;
        for(int i =0; i < nums.size(); i ++){
            int cur = nums[i];
            int curLen = 0; 
            while(mapp.find(cur)!=mapp.end()){
                mapp.erase(cur);
                cur--;
                curLen++; 
            }
            cur = nums[i] + 1;
            while(mapp.find(cur)!=mapp.end()){
                mapp.erase(cur);
                cur++;
                curLen++; 
            }
            max_len = max(max_len, curLen);
        }
        return max_len; 
        
        
    }
};