46. Permutations (Recursion, DP)

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].

思路：遍历数组，对于该字母，它可选择与它之后的字母交换或者是不交换=>带回溯的递归

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        result.clear();
        dfs(num,0);
        return result;

    }
    void dfs(vector<int> num, int depth)
    {
        if(depth == num.size()-1)
        {
            result.push_back(num);
            return;
        }
        dfs(num,depth+1);
        int temp = num[depth];
        for(int i = depth+1;i< num.size(); i++)
        {
            num[depth] = num[i];
            num[i] = temp;
            dfs(num,depth+1);
            num[i] = num[depth];
            num[depth] = temp;
        }
    }

private:
    vector<vector<int> >  result;
};