Longest Consecutive Sequence

Q:

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.

A:

先遍历一遍，使用hash_map存储所有的数字，并把所有的value初始化为0，value在后续计算最长连续数字的时候会用到。

重新遍历一遍，计算以当前数字为尾的最长连续数字的长度，如果value非0直接跳过，表示该数字已经使用过，否则查看该数字-1对应的数字是否在hash_map中，如果在，加上该数字对应的value即可。说的不是很清楚，直接贴代码，很容易看懂。

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        unordered_map<int, int> conse_num;
        for (int i = 0; i < num.size(); ++i) {
            conse_num[num[i]] = 0;
        }
        int max_conse = 0;
        for (int i = 0; i < num.size(); ++i) {
            if (conse_num[num[i]] != 0) continue;
            int cur = num[i];
            int pre = cur - 1;
            conse_num[cur] = 1;
            while (conse_num.find(pre) != conse_num.end()) {
                if (conse_num[pre] != 0) {
                    conse_num[cur] += conse_num[pre];
                    break;
                } else {
                    conse_num[pre] = 1;
                    conse_num[cur]++;
                    pre--;
                }
                
            }
            max_conse = max(max_conse, conse_num[cur]);
        }
        return max_conse;
    }
};