[LeetCode-128] 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.

 

用unordered_set装起来，来回找就ok了~

 

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int max_length = 0, tmp_length = 0, tmp_num = 0;
        unordered_set<int> num_set;
        num_set.clear();
        
        for (vector<int>::iterator iter = num.begin();
            iter != num.end(); ++iter) {
                num_set.insert(*iter);
        }
        for (vector<int>::iterator iter = num.begin();
            iter != num.end(); ++iter) {
            if (num_set.end() != num_set.find(*iter)) {
                tmp_length = 1;
                // find
                tmp_num = *iter;
                for (;;) {
                    ++tmp_num;
                    if (num_set.end() != num_set.find(tmp_num)) {
                        num_set.erase(tmp_num);
                        ++tmp_length;
                    } else {
                        break;
                    }
                }
                tmp_num = *iter;
                for (;;) {
                    --tmp_num;
                    if (num_set.end() != num_set.find(tmp_num)) {
                        num_set.erase(tmp_num);
                        ++tmp_length;
                    } else {
                        break;
                    }
                }
                if (tmp_length > max_length) {
                    max_length = tmp_length;
                }
            }
        }
        return max_length;
    }
};