代码随想录算法训练营第29天|491.递增子序列 46.全排列 47.全排列 II

491.递增子序列

不能排序：排序会改变元素顺序，增加结果。 

终止条件可以不写的原因：startIndex最终会走到size之外自动终止。

class Solution {
private:
    vector> res;
    vector path;
    void backtracking(vector& nums, int startIndex) {
        if (path.size() > 1) res.push_back(path);
        unordered_set uset;
        for (int i = startIndex; i < nums.size(); i++) {
            if ((!path.empty() && nums[i] < path.back()) || uset.find(nums[i]) != uset.end()) {
                continue;
            }
            uset.insert(nums[i]);
            path.push_back(nums[i]);
            backtracking(nums, i + 1);
            path.pop_back();
        }
    }
public:
    vector> findSubsequences(vector& nums) {
        path.clear();
        res.clear();
        backtracking(nums, 0);
        return res;
    }
};

 for中if条件两种情况需要仔细思考。

46.全排列

class Solution {
private:
    vector path;
    vector> res;
    void backtracking(vector& nums, vector& used) {
        if (path.size() == nums.size()) {
            res.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);
            path.pop_back();
            used[i] = false;
        }

    }
public:
    vector> permute(vector& nums) {
        path.clear();
        res.clear();
        vector used(nums.size(), false);
        backtracking(nums, used);
        return res;
    }
};

47.全排列 II

class Solution {
private:
    vector path;
    vector> res;
    void backtracking(vector& nums, vector& used) {
        if (path.size() == nums.size()) {
            res.push_back(path);
            return;
        }
        for (int i = 0; i < nums.size(); i++) {
            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;
            }
        }
    }
public:
    vector> permuteUnique(vector& nums) {
        path.clear();
        res.clear();
        sort(nums.begin(), nums.end());
        vector used(nums.size(), false);
        backtracking(nums, used);
        return res;
    }
};