leetcode 46. Permutations

Given a collection of distinct integers, return all possible permutations.

Example:

Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

思路一：二话不说，经典题， 回溯。

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        int len = nums.size();
        vector<bool> visited(len, false);
        vector<vector<int> > res;
        vector<int> tmp;
        dfs(0, len, nums, tmp, res, visited);
        return res;
    }
private:
    void dfs(int start, int len, vector<int> &nums, vector<int> &v, vector<vector<int> > &res, vector<bool> &visited) {
        if (start == len) {
            res.push_back(v);
            return ;
        }
        for (int i = 0; i < len; i++) {
            if (visited[i] == false) {
                visited[i] = true;
                v.push_back(nums[i]);
                dfs(start + 1, len, nums, v, res, visited);
                v.pop_back();
                visited[i] = false;
            }
        }
    }
};

思路二：

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        int len = nums.size();
        vector<vector<int> > res;
        dfs(0, len, nums, res);
        return res;
    }
private:
    void dfs(int start, int len, vector<int> &nums, vector<vector<int> > &res) {
        if (start == len) {
            res.push_back(nums);
            return ;
        }
        for (int i = start; i < len; i++) {
            swap(nums[start], nums[i]);
            dfs(start + 1, len, nums, res);
            swap(nums[start], nums[i]);
        }
    }
};