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  • [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.

    我的思路

    这道题在 LeetCode 上通过率也就 30% 不到,还标记为 hard 了,其实题目并不难。

    使用排序,当然可以解决这个问题,但是时间复杂度为 O(nlgn) ,显然不符合我们的题意。这道题我们需要考虑使用 hashmap ,以下是我 AC 后的代码,注意 LeetCode 支持c++11。

    /**
     * Zhou J
     */
    
    class Solution 
    {
    public:
        int longestConsecutive(vector<int> &num) 
        {
            unordered_map<int, int> hs;
            for (const auto &ival : num)
            {
                hs.insert(make_pair(ival, 0));
            }
            
            int maxl = 1;
            
            for (const auto &ival : num)
            {
                if (hs.find(ival)->second == 1)
                {
                    continue;
                }
                
                // initialize the current_max before every loop
                int current_max = 1;
                
                // record the right side
                int tmp = ival;
                while(hs.count(tmp + 1))
                {
                    current_max ++;
                    tmp ++;
                    hs[tmp] = 1;
                }
                
                // record the left side
                tmp = ival;
                while(hs.count(tmp - 1))
                {
                    current_max ++;
                    tmp --;
                    hs[tmp] = 1;
                }
                
                // update the returned maxl
                maxl = max(current_max, maxl);
            }
            
            return maxl;
        }
    };
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  • 原文地址:https://www.cnblogs.com/jianxinzhou/p/4214153.html
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