Leetcode|组合变种|491. 递增子序列（first索引+跳过非相邻重复元素）

class Solution {
private:
    int size;
    vector<vector<int>> solution;
    vector<int> path;
public:
    void backtrack(vector<int>& nums, int first) {
        if (path.size() >= 2) 
            solution.emplace_back(path);
        unordered_set<int> set;
        // 1.first索引避免乱序重复
        for (int i = first; i < size; i++) {
            // 2.跳过重复元素(set.count(nums[i])代替nums[i-1] == nums[i])
            if (set.count(nums[i])) continue;
            if (!path.empty() && path.back() > nums[i]) continue;
            set.insert(nums[i]);
            path.emplace_back(nums[i]);
            backtrack(nums, i + 1);
            path.pop_back();
        }
    }
    vector<vector<int>> findSubsequences(vector<int>& nums) {
        size = nums.size();
        backtrack(nums, 0);
        return solution;
    }
};