46. Permutations

image.png

解法一：普通的回溯

class Solution {
public:
    vector> permute(vector& nums) {
        vector> result;
        if(nums.empty()) return result;
        vector list;
        dfs(list, result, nums);
        return result;
    }
private:
    void dfs(vector& list, vector>& result, vector 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> permute(vector& nums) {
        vector> result;
        if(nums.empty()) return result;
        vector list;
        vector used(nums.size(), false);
        dfs(list, result, nums, used);
        return result;
    }
private:
    void dfs(vector& list, vector>& result, vector nums, vector 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> permute(vector& nums) {
        vector> result;
        if(nums.empty()) return result;
        vector list;
        permuteHelper(result, nums, 0);
        return result;
    }
private:
    void permuteHelper(vector>& result, vector& 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/

image.png