19.2.7 [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]
]

class Solution {
public:
    void build(vector<bool>visited, vector<int>nums, vector<vector<int>>&ans,vector<int>now) {
        int flag = false;
        int n = nums.size();
        for(int i=0;i<n;i++)
            if (!visited[i]) {
                visited[i] = true;
                now.push_back(nums[i]);
                build(visited, nums, ans, now);
                visited[i] = false;
                now.pop_back();
                flag = true;
            }
        if (!flag)ans.push_back(now);
    }
    vector<vector<int>> permute(vector<int>& nums) {
        vector<bool>visited(nums.size(), false);
        vector<vector<int>>ans;
        build(visited, nums, ans, vector<int>());
        return ans;
    }
};

一开始的暴力方法↑ 不出所料很慢

换了一种也没好多少

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>>ans;
        ans.push_back(vector<int>(1, nums[0]));
        for (int i = 1; i < nums.size(); i++) {
            vector<vector<int>>tmp = ans;
            ans.clear();
            for (int j = 0; j < tmp.size(); j++) {
                vector<int>now = tmp[j];
                for (int k = 0; k <= now.size(); k++) {
                    now.insert(now.begin() + k, nums[i]);
                    ans.push_back(now);
                    now.erase(now.begin() + k);
                }
            }
        }
        return ans;
    }
};

（算了算了