Problem

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Clarification

Your algorithm should run in O(n) complexity.

Example

Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Note

先排序，然后用count[]数组记录每一位上连续序列的长度，每次循环更新最大值存为max。

Solution

public class Solution {
    public int longestConsecutive(int[] num) {
        Arrays.sort(num);
        int[] count = new int[num.length];
        count[0] = 1;
        int max = 1;
        for (int i = 1; i < num.length; i++) {
            if (num[i] == num[i-1]) count[i] = count[i-1];
            else if (num[i] == num[i-1]+1) count[i] = count[i-1]+1;
            else count[i] = 1;
            max = Math.max(max, count[i]);
        }
        return max;
    }
}