Longest Consecutive Sequence

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.

对于线性表的题目，如果要求复杂度是O(n)的，我只能说，请考虑hash!!!!!!!!!!!!!!!!!!!!!!!

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_map<int, bool> used;
        int i,j;
        for(i = 0; i < num.size(); i++){
            used[num[i]] = false;
        }
        
        int longest = 0;
        for(i =0; i < num.size(); i++){
            if (used[num[i]]) continue;
            
            int length = 1;
            
            used[num[i]] = true;
            
            for(j = num[i] + 1; used.find(j) != used.end(); ++j){
                used[j] = true;
                ++length;
            }
            
            for(j = num[i]-1; used.find(j) != used.end(); --j){
                used[j] = true;
                ++length;
            }
            
            longest = max(longest, length);
        }
        return longest;
    }
};