leetcode -- Longest Consecutive Sequence

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.

 

large data TLE

 1 public class Solution {
 2     public int longestConsecutive(int[] num) {
 3         // Start typing your Java solution below
 4         // DO NOT write main() function
 5         int result = 0;
 6         HashSet<Integer> con = new HashSet<Integer>();
 7         for(int i = 0; i < num.length; i++){
 8             int tmpLen = 1;
 9             int val = num[i];
10             con.add(val);
11             while(con.contains(--val)){
12                 tmpLen ++;
13             }
14             val = num[i];
15             while(con.contains(++val)){
16                 tmpLen ++;
17             }
18             
19             if(tmpLen > result){
20                 result = tmpLen;
21             }
22         }
23         
24         return result;
25     }
26 }

 refactor：

1.使用HashSet来去重

2.遍历数组，循环查看每个元素的相邻元素：

i.检查num-1是否在Set中(loop)，如在则从Set中去除

i.检查num+1是否在Set中(loop)，如在则从Set中去除

 1 public int longestConsecutive(int[] num) {
 2         // Start typing your Java solution below
 3         // DO NOT write main() function
 4         int result = 0;
 5         HashSet<Integer> con = new HashSet<Integer>();
 6         for(int i = 0; i < num.length; i++){
 7             con.add(num[i]);
 8         }
 9         
10         for(int i = 0; i < num.length; i++){
11             int tmpLen = 1;
12             int val = num[i];
13             while(con.contains(--val)){
14  con.remove(val);                
15                 tmpLen ++;
16             }
17             val = num[i];
18             while(con.contains(++val)){
19                 con.remove(val);
20                 tmpLen ++;
21             }
22             con.remove(num[i]);
23             
24             if(tmpLen > result){
25                 result = tmpLen;
26             }
27         }
28         
29         return result;
30     }