【leetcode】128. Longest Consecutive Sequence

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.
翻译：
在一个随机取值的数组中找到最大的连续区间长度。

思路：

这题一开始写的时候TLE了，后来参考leetcode上的一个大神的代码（不得不说太简洁了），我看了一下和我的思路差不多，可能是我的代码中有区间的删除和合并，这个有些耗时。总体思路：遍历数组时总共只有以下四种情况：

1) neither i+1 nor i-1 has been seen: m[i]=1;

2) both i+1 and i-1 have been seen: extend m[i+m[i+1]] and m[i-m[i-1]] to each other;

3) only i+1 has been seen: extend m[i+m[i+1]] and m[i] to each other;

4) only i-1 has been seen: extend m[i-m[i-1]] and m[i] to each other.

代码

class Solution {
public:
int longestConsecutive(vector<int> &nums) {
map<int, int> m;
int r = 0;
for (int i=0;i<nums.size();i++) {
if (m[nums[i]]) continue;
r = max(r, m[nums[i]] = m[nums[i] + m[nums[i] + 1]] = m[nums[i]- m[nums[i] - 1]] = m[nums[i] + 1] + m[nums[i] - 1] + 1);
}
return r;
}
};

结果：