[LeetCode]Longest Consecutive Sequence

题目描述:(链接)

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:

题目要求时间复杂度O(n)，于是想到了哈希表，记录每个元素的使用情况，求出最长的长度。

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_map<int, bool> used;
        int longest = 0;
        
        for (auto i : nums) {
            used[i] = false;
        }
        
        for (auto i : nums) {
            if (used[i]) { continue; }
            int length = 1;
            for (int j = i + 1; used.find(j) != used.end(); ++j) {
                used[j] = true;
                ++length;
            }
            
            for (int j = i - 1; used.find(j) != used.end(); --j) {
                used[j] = true;
                ++length;
            }
            
            if (length > longest) {
                longest = length;
            }
        }
        
        return longest;
    }
};