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.
题目要求我们用O(n)的时间复杂度来解决,我们可以采用并查集的思想,当扫描到一个元素,我们从当前元素的两边开始处理,如果连续的都进行标记,同时维护一个最大值,再次扫描的时候如果被标记过的就可以跳过,这样我们就可以在O(n)的时间复杂度下找到最长的连续序列。代码如下:
public class Solution { public int longestConsecutive(int[] nums) { if(nums == null) return 0; HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>(); int max = 1; for(int i : nums) hm.put(i, 0); for(int i = 0; i < nums.length; i++) { int count = 1; int left = -1; int right = 1; if(hm.get(nums[i]) == 1) continue; hm.put(nums[i], 1); while(hm.containsKey(nums[i] + left)) { hm.put(nums[i] + left, 1); count ++; left --; } while(hm.containsKey(nums[i] + right)) { hm.put(nums[i] + right, 1); count ++; right ++; } max = Math.max(max, count); } return max; } }