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;}
};