Leetcode -- 491. Increasing Subsequences

题目：

Given an integer array, your task is to find all the different possible increasing subsequences of the given array, and the length of an increasing subsequence should be at least 2 .
Example:

Input: [4, 6, 7, 7]Output: [[4, 6], [4, 7], [4, 6, 7], [4, 6, 7, 7], [6, 7], [6, 7, 7], [7,7], [4,7,7]]

Note:
The length of the given array will not exceed 15.
The range of integer in the given array is [-100,100].
The given array may contain duplicates, and two equal integers should also be considered as a special case of increasing sequence.

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思路：
这是一个典型的深搜问题，遍历到最后的结点时，回溯。

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C++ Codes:

class Solution{public:    vector<vector<int>> findSubsequences(vector<int>& nums) {        vector<vector<int>> res;        vector<int> tmp;        tmp.clear();        dfs(res,nums,tmp,0);        return res;    }    void dfs(vector<vector<int>>& res, vector<int>& nums, vector<int>& tmp, int index)    {        if(tmp.size()>=2)            res.push_back(tmp);        unordered_set<int> s; //基于hash的无序集合，访问速度更快        for(int i=index;i<nums.size();i++)        {            if((tmp.empty()||tmp.back()<=nums[i])&&s.find(nums[i]) == s.end()) //nums[i]不在s中，返回s.end()            {                tmp.push_back(nums[i]);                dfs(res,nums,tmp,i+1);                tmp.pop_back(); //回溯                s.insert(nums[i]); //将nums[i]加入到s中            }        }    }};