46. Permutations

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解法一：普通的回溯

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> result;
        if(nums.empty()) return result;
        vector<int> list;
        dfs(list, result, nums);
        return result;
    }
private:
    void dfs(vector<int>& list, vector<vector<int>>& result, vector<int> nums){
        if(list.size() == nums.size()){
            result.push_back(list);
            return;
        }
        for(int i = 0; i < nums.size(); i++){
            if(find(list.begin(), list.end(), nums[i]) != list.end()){
                continue;
            }
            list.push_back(nums[i]);
            dfs(list, result, nums);
            list.pop_back();
        }
    }
    
};

或使用数组used[]来记录

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

        }
    }
    
};

解法二：使用交换的思想

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> result;
        if(nums.empty()) return result;
        vector<int> list;
        permuteHelper(result, nums, 0);
        return result;
    }
private:
    void permuteHelper(vector<vector<int>>& result, vector<int>& nums, int begin){
        if(begin >= nums.size()){
            result.push_back(nums);
            return;
        }
        for(int i = begin; i < nums.size(); i++){
            swap(nums[i], nums[begin]);
            permuteHelper(result, nums, begin+1);
            swap(nums[i], nums[begin]);
        }
    }
    
};

https://www.geeksforgeeks.org/write-a-c-program-to-print-all-permutations-of-a-given-string/

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