Longest Consecutive Sequence

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

Clarification
Your algorithm should run in O(n) complexity.

类似题目 ​​最长递增子序列（LIS）​​

思路就是：
1.将num存到unordered_map(或者unordered_set)中；
2.
- 对于num的每个数（没有访问过的）n
- 在unordered_map中查找n+1,n-1在不在，循环进行这一步，查找到的数在unordered_map中去掉，可以统计出n导出的连续的数的长度len，如果len>maxLen,更新maxLen。

需要注意几点：
1.unordered_map、unordered_set和hash_map、hash_set
见我的博客​​数据结构：hash_map原理​​ ​​C++中map、set、hash_map、hash_set、unordered_map、unordered_set通俗辨析​​

2.以前看一直以为erase函数的参数只能是迭代器，其实erase的参数还可以是key_type
​​​http://www.cplusplus.com/reference/map/map/erase/​​

std::map::erase
(1) 
iterator  erase (const_iterator position);
(2) 
size_type erase (const key_type& k);
(3) 
iterator  erase (const_iterator first, const_iterator last);

For the key-based version (2), the function returns the number of elements erased, 
which in map containers is at most 1.

class Solution {
public:
    /**
     * @param nums: A list of integers
     * @return an integer
     */
    int longestConsecutive(vector<int> &num) {
        // write you code here

        unordered_map<int,bool> tmpNum;

        for(int n:num){
            tmpNum[n]=false;
        }

        int maxLen=0;

        for(int n:num){
            if(tmpNum[n]) continue;

            int len=0;
            queue<int> Q;
            Q.push(n);
            while(!Q.empty()){
                int i=Q.front();
                Q.pop();

                /*tmpNum.find(i)->second==false 不判断会出现死循环*/

                if(tmpNum.find(i)!=tmpNum.end() && tmpNum.find(i)->second==false){
                   len++;
                   tmpNum[i]=true;
                   Q.push(i+1);
                   Q.push(i-1);
               }
            }

            if(len>maxLen){
                maxLen=len;
            }

        }

        return maxLen;
    }
};

理解成图的深度搜索。数字是点，邻边是相邻的数字。

class Solution {
public:
    /**
     * @param nums: A list of integers
     * @return an integer
     */
    int longestConsecutive(vector<int> &num) {
        // write you code here

        unordered_map<int,bool> tmpNum;

        for(int n:num){
            tmpNum[n]=false;
        }

        int maxLen=0;

        for(int n:num){

            if(tmpNum[n]) continue;

            int len=0;

            int up=n;
            while(tmpNum.find(up)!=tmpNum.end()){
                //tmpNum.erase(up);
                len++;
                tmpNum[up]=true;

                up++;
            }

            int down=n-1;
            while(tmpNum.find(down)!=tmpNum.end()){
                //tmpNum.erase(down);
                len++;
                tmpNum[down]=true;

                down--;
            }

            if(len>maxLen){
                maxLen=len;
            }

        }

        return maxLen;
    }
};