Leetcode -- 491. Increasing Subsequences

題目：

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：
這是一個典型的深搜問題，遍歷到最後的結點時，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; //基於hash的無序集合，訪問速度更快
        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]); //將nums[i]加入到s中
            }
        }
    }
};