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.

 

hash table查找的时间复杂度O(1)，可以是时间复杂度降为O(n)。

1 class Solution { 2 public: 3     int longestConsecutive(vector
     
      & nums) { 4         if(nums.size()<1) return 0; 5         int result=0; 6         unordered_map
      
        showed; 7         for(int i=0;i