【Longest Consecutive Sequence】cpp

题目：

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.

代码：

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        // hashmap record if element in num is visited
        std::map<int,bool> visited;
        for(std::vector<int>::iterator i = num.begin(); i != num.end(); ++i)
        {
            visited[*i] = false;
        }
        // search the longest consecutive
        unsigned int longest_global = 0;
        for(std::vector<int>::iterator i = num.begin(); i != num.end(); ++i)
        {
            if(visited[*i]) continue;
            unsigned int longest_local = 0;
            for(int j = *i+1; visited.find(j) != visited.end(); ++j)
            {
                visited[j] = true;
                ++longest_local;
            }
            for(int j = *i-1; visited.find(j) != visited.end(); --j)
            {
                visited[j] = true;
                ++longest_local;
            }
            longest_global = std::max(longest_global, longest_local+1);
        }
        return longest_global;
    }
};

Tips:

1. 要想O(n), 而且无序，只能结合hashmap

2. 这里需要明确的一个逻辑是，通过hashmap前后访问，可以把包含当前元素的最大连同序列都找出来；而且访问过的元素不用再访问。