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/