128. 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.

class Solution {
public:
    int longestConsecutive(vector& nums) {
        if(nums.size()<=0)
          return 0;
        unordered_set exsitSet;
        unordered_set visitedSet;
        for(int i=0;i