128. Longest Consecutive Sequence Hard

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：本题要求复杂度为O(n)，并且注意如果数组中有相同的数，那么他们不算连续的数。这样，我们使用一个map来存储原数组，首先就消除了重复的数， 将数组中的值同时作为map的first以及second，这样遍历map是就是从小到大的顺序，不需要排序，只需要第一次遍历数组，为O(n)，第二次遍历map也为O(n)就可以了。

class Solution {public:    int longestConsecutive(vector<int>& nums) {        if (nums.size() == 0) {            return 0;        }        map<int, int>hash;        for (int i = 0; i < nums.size(); i++) {            hash[nums[i]] = nums[i];        }        map<int, int>::iterator it = hash.begin();        int count = 1;        int max = 1;        while ((++it) != hash.end()) {            it--;            if (it -> second == (++it) -> second - 1) {                count++;            } else {                count = 1;            }            if (count > max) {                max = count;            }        }         return max;    }};