Longest Consecutive Sequence 解答

Question

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.

Solution

一开始尝试用segment tree, heap等，发现时间复杂度不能达到线性。

后来参考别人答案，发现用HashSet计数的方法。

程序比较简单。

1. 遍历一遍数组，将所有元素存入set

2. 遍历数组。对于当前元素x，看x+1和x-1是否在set中。如果在，则remove，并且一直找到这个区间的边界。

 public class Solution {
     public int longestConsecutive(int[] nums) {
         Set<Integer> set = new HashSet<Integer>();
         for (int i : nums) {
             set.add(i);
         }
         int max = 0;
         int count;
         for (int i : nums) {
             if (set.isEmpty()) {
                 break;
             }
             count = 1;
             int right = i + 1;
             int left = i - 1;
             set.remove(i);
             // find right
             while (set.contains(right)) {
                 set.remove(right);
                 count++;
                 right++;
             }
             // find left
             while (set.contains(left)) {
                 set.remove(left);
                 count++;
                 left--;
             }
             max = Math.max(max, count);
         }
         return max;
     }
 }