LeetCode128 Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence. （Hard）

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.

分析：

要做到O(n)的时间复杂度，所以排序是不能用了。

用空间换时间，建立一个哈希表。对每一个元素把上下区间求出来，然后从hash表中删除。

代码：

1 class Solution {
2 public:
3     int longestConsecutive(vector<int>& nums) {
4         unordered_map<int, bool> hash;
5         for (int i = 0; i < nums.size(); ++i) {
6             hash[nums[i]] = true;
7         }
8         int result = 1;
9         for (int i = 0; i < nums.size(); ++i) {
10             int cur = nums[i];
11             hash.erase(cur);
12             int low = cur - 1, high = cur + 1;
13             while (hash.find(low) != hash.end()) {
14                 hash.erase(low);
15                 low--;
16             }
17             while (hash.find(high) != hash.end()) {
18                 hash.erase(high);
19                 high++;
20             }
21             result = max(result, high - low - 1);
22         }
23         return result;
24     }
25 };