【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：

1. 将所有数字想放进一个集合set中

2. 遍历原数组nums，对于当前便利的数字，在set中查找是否有连续的， 即先逐步累加看累加的当前数字是否在set中，在则累加当前curLen，并从集合中删除（避免反复查看）； 不在则同样方式逐步减一在集合中看是否存在。

3. 最后将此次便利的curLen 与maxLen 进行比较更新。

代码如下:

public class Solution {    public int longestConsecutive(int[] nums) {        HashSet<Integer> set = new HashSet<Integer>();        //add all the num into a set        for(int num: nums)            set.add(num);        int maxLen = 0;        //for each of the num in nums, see if there consecutive number        for(int num: nums){            int cur = num;            int curLen = 0;//keep track of the current Length            while(set.contains(cur)){//count all the numbers after cur                set.remove(cur);                cur++;                curLen++;                            }            cur = num -1;            while(set.contains(cur)){//count all the numbers before cur                set.remove(cur);                cur --;                curLen++;            }            maxLen = Math.max(maxLen, curLen);//update global max        }        return maxLen;    }}