LeetCode Longest Consecutive Sequence

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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.

 

由於演算法複雜度是O(n) ，所以不能直接排序。可以把數組存入hashset,考察每個元素的相鄰元素是否在set內，把考察過的元素刪除。

 

 1 public class Solution { 2     public int longestConsecutive(int[] num) { 3             HashSet<Integer> hashSet; 4          5             if (num.length==0) { 6                 return 0; 7             } 8             if (num.length==1) { 9                 return 1;10             }11             hashSet=new HashSet<>();12             13             for (int i = 0; i < num.length; i++) {14                 hashSet.add(num[i]);15             }16             int res=1;17             for (int i = 0; i < num.length; i++) {18                 int tmplen=1;19                 int tmpnum=num[i]+1;20                 while (hashSet.contains(tmpnum)) {21                     tmplen++;22                     hashSet.remove(tmpnum);23                     tmpnum++;24                 }25                 tmpnum=num[i]-1;26                 while (hashSet.contains(tmpnum)) {27                     tmplen++;28                     hashSet.remove(tmpnum);29                     tmpnum--;30                 }31                 if (tmplen>res) {32                     res=tmplen;33                 }34                 35             }36             return res;37     }38 }

 

LeetCode Longest Consecutive Sequence