[Leetcode] Longest Consecutive Sequence

题目：

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：由于没有sort，又要求O(n)内完成，想到用hashmap实现。具体做法是把所有元素放到一个hashset中，从两边扩展。

class Solution {public:    int longestConsecutive(vector<int> &num) {        int result = 0;        unordered_set<int> num_set;        for (int i = 0; i < (int)num.size(); ++i) {            num_set.insert(num[i]);        }        while (!num_set.empty()) {            int current = *num_set.begin();   //arbitrarily pick one            num_set.erase(num_set.begin());            int len = 1;            unordered_set<int>::iterator it;            for (int i = current - 1; ; --i) {   //expand to left                it = num_set.find(i);                if (it != num_set.end()) {                    len++;                    num_set.erase(it);                } else {                    break;                }            }            for (int i = current + 1; ; ++i) {   //expand to right                it = num_set.find(i);                if (it != num_set.end()) {                    len++;                    num_set.erase(it);                } else {                    break;                }            }            result = result > len ? result : len;   //update the length        }        return result;    }};

总结：每个元素被插入一次，删除一次。复杂度为O(n).