129.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.

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Step1:先把所有的元素都存入到set中；
Step2:遍历set中的元素，针对这个元素把所有连续比它大的元素都删除，然后再把所有比它小的元素都删除，并记录下个数；
Step3:继续遍历set中剩下的元素，重复Step2。

/**
* 给定一个未排序的数组，求该数组中元素连续的最大长度。
* Given [100, 4, 200, 1, 3, 2],
* The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
* 思路：
* Step1:先把所有的元素都存入到set中；
* Step2:遍历set中的元素，针对这个元素把所有连续比它大的元素都删除，然后再把所有比它小的元素都删除，并记录下个数；
* Step3:继续遍历set中剩下的元素，重复Step2。
* @date 20160512
*/
public int longestConsecutive(int[] nums) {

int len = nums.length;
if(len <= 0){
return 0;
}
int result = 1;
Set<Integer> set = new HashSet<Integer>(len);
for(int i=0;i<len;i++){
set.add(nums[i]);
}

Iterator<Integer> it = set.iterator();
while(!set.isEmpty()){
it = set.iterator();
int element = it.next();
set.remove(element);
int next = element+1;//比element大的元素
int pre = element-1;
while(set.contains(next)){
set.remove(next);
next++;
}
while(set.contains(pre)){
set.remove(pre);
pre--;
}
if(result < next-pre-1){
result = next-pre-1;
}
}
return result;
}