491. 非递减子序列

class Solution {
public:

    vector<int> path;
    vector<vector<int>> result;
    void BackTracking(vector<int>& nums, int index){
        if(path.size()>=2){
            result.push_back(path);
        }
        unordered_set<int> usedSet;
        for (int i=index; i<nums.size(); i++){
            if (!path.empty() && nums[i]<path.back() || usedSet.find(nums[i])!=usedSet.end())
                continue;
            usedSet.insert(nums[i]);
            path.push_back(nums[i]);
            BackTracking(nums, i+1);
            path.pop_back();
        }
    }
    vector<vector<int>> findSubsequences(vector<int>& nums) {
        BackTracking(nums, 0);
        return result;
    }
};

46. 全排列

class Solution {
public:
    vector<int> path;
    vector<vector<int>> result;
    void BackTracking(vector<int>& nums, vector<bool>& used){
        if (path.size() == nums.size()){
            result.push_back(path);
            return ;
        }
        for (int i=0; i<nums.size(); i++){
            if (used[i]==true) continue;
            path.push_back(nums[i]);
            used[i]=true;
            BackTracking(nums, used);
            used[i]=false;
            path.pop_back();
        }
    }
    vector<vector<int>> permute(vector<int>& nums) {
        result.clear();
        path.clear();
        vector<bool> used(nums.size(), false);
        BackTracking(nums, used);
        return result;
    }
};

组合不强调顺序，排列强调顺序。

47.全排列 II

class Solution {
public:
    vector<int> path;
    vector<vector<int>> result;
    void BackTracking(vector<int>& nums, vector<bool>& used){
        if (nums.size() == path.size()){
            result.push_back(path);
            return ;
        }
        for (int i=0; i<nums.size(); i++){
            // used[i - 1] == true，说明同一树枝nums[i - 1]使用过
            // used[i - 1] == false，说明同一树层nums[i - 1]使用过
            // 如果同一树层nums[i - 1]使用过则直接跳过
            if (i > 0 && nums[i] == nums[i - 1] && used[i - 1] == false) {
                continue;
            }
            if (used[i] == false) {
                used[i] = true;
                path.push_back(nums[i]);
                BackTracking(nums, used);
                path.pop_back();
                used[i] = false;
            }
        }
    }
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        vector<bool> used(nums.size(), false);
        sort(nums.begin(), nums.end());
        BackTracking(nums, used);
        return result;
    }
};