【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;
}
}