【leetcode】Permutations (middle)

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

 

求没有重复数字的全排列

 

思路：用的标准回溯法，一次AC 

class Solution {

public:

    vector<vector<int> > permute(vector<int> &num) {

        vector<vector<int>> ans;

        if(num.empty())

        {

            return ans;

        }



        vector<int> X(num.size());

        vector<vector<int>> S(num.size());

        int k = 0;

        S[k] = num;



        while(k >= 0)

        {

            while(!S[k].empty())

            {

                X[k] = S[k].back();

                S[k].pop_back();

                if(k < num.size() - 1)

                {

                    k++;

                    S[k] = num;

                    for(int i = 0; i < k; i++)

                    {

                        vector<int>::iterator  it;

                        if((it = find(S[k].begin(), S[k].end(), X[i])) != S[k].end())

                        {

                            S[k].erase(it);

                        }

                    }

                }

                else

                {

                    ans.push_back(X);

                }

            }

            k--;

        }



        return ans;        

    }

};

 

网上也有用递归的

class Solution {

public:

    vector<vector<int> > permute(vector<int> &num) {

        vector<vector<int> > result;



        permuteRecursive(num, 0, result);

        return result;

    }



    // permute num[begin..end]

    // invariant: num[0..begin-1] have been fixed/permuted

    void permuteRecursive(vector<int> &num, int begin, vector<vector<int> > &result)    {

        if (begin >= num.size()) {

            // one permutation instance

            result.push_back(num);

            return;

        }



        for (int i = begin; i < num.size(); i++) {

            swap(num[begin], num[i]);

            permuteRecursive(num, begin + 1, result);

            // reset

            swap(num[begin], num[i]);

        }

    }

};