LeetCode OJ

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.

解题思路：

用unordered_map来记录数字是否在数组中。 然后枚举每个数，看其相邻的数在不在unorded_map中，相邻的数在数组里的话，每个数之多访问一次；相邻的数不在数组里的话，枚举会中断。所以设哈希复杂度为O(1)的话，这个方法是严格的O(n)。

代码：

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_map<int, bool> dict;
        //init
        for (int i = 0; i < num.size(); i++) {
            dict[num[i]] = false;
        }

        int ans = 0;
        for (int i = 0; i < num.size(); i++) {
            if (dict[num[i]] == false) {
                int cnt = 0;
                //枚举相邻的数，看是否在字典中
                for (int j = num[i]; dict.count(j); j++, cnt++) {
                    dict[j] = true;
                }
                for (int j = num[i] - 1; dict.count(j); j--, cnt++) {
                    dict[j] = true;
                }
                ans = max(ans, cnt);
            }
        }
        return ans;
    }
};