128. 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.

解题思路

consecutive	
英[kənˈsekjətɪv], 美[kənˈsɛkjətɪv]
adj. 连续的，连贯的; [语] 表示结果的;

见程序注释