Wednesday, April 8, 2015

LeetCode [128] Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
Your algorithm should run in O(n) complexity.
Example:
Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_map<int, int> mp;//number, maxLen
        int ret = 0;
        for(auto n:nums)
        {
            if(mp.count(n)==0)
            {
                int len = 1;
                if(mp.count(n-1)!=0 && mp.count(n+1)!=0){
                    int left = n-mp[n-1];
                    int right = n+mp[n+1];
                    len = mp[n-1]+mp[n+1]+1;      
                    mp[left] = len;
                    mp[right] = len;
                }else if(mp.count(n-1)!=0){
                    len = mp[n-1]+1;
                    mp[n+1-len] = len;
                }else if(mp.count(n+1)!=0){
                    len = mp[n+1]+1;
                    mp[len+n-1] = len;
                }
                mp[n] = len;
                ret = max(ret, len);
            }
        }
        return ret;
    }
};

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_map<int, int> hash;//number, length
        int ret = 0;
        for(auto n:nums){
            if(hash.count(n)) continue;
            int nl = 0, nr = 0;
            if(hash.count(n-1)) nl = hash[n-1];
            if(hash.count(n+1)) nr = hash[n+1];
            int len = nl+nr+1;
            hash[n] = len;
            hash[n-nl] = len;
            hash[n+nr] = len;
            ret = max(ret, len);
        }
        return ret;
    }
};

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_set<int> ns(nums.begin(), nums.end());
        int ret = 0;
        for(auto n:ns){
            if(ns.count(n-1)==0){
                int tmp = 0;
                while(ns.count(n+tmp)){
                    tmp++;
                }
                ret = max(ret, tmp);
            }
        }
        return ret;
    }
};

No comments:

Post a Comment