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  • LeetCode——Longest Consecutive Sequence

    LeetCode——Longest Consecutive Sequence

    Question

    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.

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    Solution

    这道题对时间复杂度要求比较高,排个序也得O(nlgn),所以想要得到O(n),那必须得用空间换时间,用上哈希表。 用哈希表的时候不要用set,因为set插入一个元素的时间复杂度是O(n),应该选择用unorder_set。

    然后遍历每个数字的时候,考虑去集合中查找有没有比这个数大1或者小1的,有的话就继续查找。

    我们会发现,遍历4的时候,得到的集合是1, 2, 3, 4, 遍历1的时候也是得到1, 2, 3, 4, 这就重复了呀,所以再遍历4的时候,就应该把找到的元素从集合中去掉,以免重复计算,加大了时间复杂度,具体实现如下:

    class Solution {
    public:
        int longestConsecutive(vector<int> &num) {
    		unordered_set<int> table(num.begin(), num.end());
            int res = 0;
            for (int i : num) {
                if (table.find(i) == table.end()) continue;
                table.erase(i);
                int pre = i - 1;
                int next = i + 1;
                while (table.find(pre) != table.end()) table.erase(pre--);
                while (table.find(next) != table.end()) table.erase(next++);
                res = max(res, next - pre - 1);
            }
            return res;
        }
    };
    
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  • 原文地址:https://www.cnblogs.com/zhonghuasong/p/6959731.html
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