128. Longest Consecutive Sequence

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.

分析：

既然要求O(n) complexity，排序这种方法就放弃了。这里用了HashSet来保存每个数，然后从这个数开始，左右寻找是否有相邻的数。非常有新意的方法。

public class Solution {

    public int longestConsecutive(int[] num) {
        if (num == null || num.length == 0) return 0;

        HashSet<Integer> hs = new HashSet<Integer>();
        for (int i = 0; i < num.length; i++) {
            hs.add(num[i]);
        }

        int max = 0;
        for (int i = 0; i < num.length; i++) {
            if (hs.contains(num[i])) {
                int count = 1;
                hs.remove(num[i]);

                int low = num[i] - 1;
                while (hs.contains(low)) {
                    hs.remove(low);
                    low--;
                    count++;
                }

                int high = num[i] + 1;
                while (hs.contains(high)) {
                    hs.remove(high);
                    high++;
                    count++;
                }
                max = Math.max(max, count);
            }
        }
        return max;
    }
}

参考请注明出处：cnblogs.com/beiyeqingteng/