LeetCode OJ--Longest Consecutive Sequence ***

http://oj.leetcode.com/problems/longest-consecutive-sequence/

起初想的是排序，查了下O(n)的排序算法有计数排序、基数排序、桶排序。后来考虑到数据的范围不知道，并且还有可能是负数，这几种方法都不太适用。

之后想到了容器，Map、Set的查找是哈希查找，复杂度为O(1).

#include <iostream>
#include <vector>
#include <unordered_set>
using namespace std;

class Solution {
public:
unordered_set<int> dict;

int findLongestConsective(int num)
{
int ans = 0;
int temp = num;
//往上找
while(dict.find(temp)!=dict.end())
{
ans++;
dict.erase(temp);
temp++;
}
//往下找
while(dict.find(num-1)!=dict.end())
{
ans++;
dict.erase(num-1);
num--;
}
return ans;
}
int longestConsecutive(vector<int> &num) {

for(int i = 0;i<num.size();i++)
dict.insert(num[i]);

int maxLen = 0;
for(int i = 0;i<num.size();i++)
{
int t = findLongestConsective(num[i]);
if(t>maxLen)
maxLen = t;
}
return maxLen;
}
};

int main()
{
vector<int> num;
num.push_back(100);
num.push_back(4);
num.push_back(200);
num.push_back(1);
num.push_back(3);
num.push_back(2);
Solution myS;
int ans = myS.longestConsecutive(num);
cout<<ans<<endl;
}

重点是思路的转换，以及 set 的应用。