线性表-Longest Consecutive Sequence

描述

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.

思路

如果允许O（nlogn）的复杂度，那么可以先排序，可本题的要求是O（n）。
由于序列的元素是无序的，又要求O（n），首先想到用哈希表。用哈希表unordered_mapused记录每个元素是否使用过，对每个元素，以该元素为中心，往左右扩张，直到不连续为止，记录最长的长度。

class Solution {
public:
    int longestConsecutive(vector& nums) {
        unordered_map used;
        for(auto i : nums) used[i] = false;
        int longest = 0;
        for(auto i : nums){
            if(used[i]) continue;
            int length = 1;
            used[i] = true;
            for (int j = i + 1;used.find(j) != used.end();++j){
                used[j] = true;
                ++length;
            }
            for(int j = i - 1;used.find(j) != used.end(); --j){
                used[j] = true;
                ++length;
            }
            longest = max(longest,length);
        }
        return longest;
    }
};