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.

 

public class Solution {
    public int longestConsecutive(int[] nums) {
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        int longest = 0;
        int size = nums.length;
        for(int i=0;i<size;i++) {
            if(map.containsKey(nums[i])) continue;//排除重复
            map.put(nums[i],1);
            int start = nums[i];
            int end = nums[i];
            if(map.containsKey(nums[i]+1)) end = nums[i]+map.get(nums[i]+1);
            if(map.containsKey(nums[i]-1)) start = nums[i]-map.get(nums[i]-1);
            longest = Math.max(longest,end-start+1);
            map.put(start,end-start+1);//更新左边界
            map.put(end,end-start+1);//更新右边界
        }
        return longest;
    }
}