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;
    }
}