[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.

class Solution {
public:
int longestConsecutive(vector<int> &num) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
unordered_set<int> hash;

for(int i = 0; i < num.size(); i++){
hash.insert(num[i]);
}

unordered_set<int>::iterator it;
int result = 0;
int tmp_elem;
int cnt;
unordered_set<int>::iterator tmp_it;

while(!hash.empty()){
cnt = 1;
it = hash.begin();

int elem = *it;
hash.erase(it);

tmp_elem = elem + 1;
while((tmp_it = hash.find(tmp_elem)) != hash.end()){
cnt++;
tmp_elem++;
hash.erase(tmp_it);
}

tmp_elem = elem - 1;
while((tmp_it = hash.find(tmp_elem)) != hash.end()){
cnt++;
tmp_elem--;
hash.erase(tmp_it);
}
result = max(result, cnt);

}

return result;

}
};