Permutations Solutions in C++

// Source : https://oj.leetcode.com/problems/permutations/
// Author : Hao Chen
// Date   : 2014-06-21



#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
using namespace std;

/*
{ 1 2 3 }
{ 2 1 3 }
{ 3 2 1 }
{ 1 3 2 }
{ 2 3 1 }
{ 3 1 2 }
*/

/*
 *    The algroithm - Take each element in array to the first place.
 *
 *    For example: 
 *    
 *         0) initalization 
 * 
 *             pos = 0
 *             [1, 2, 3]   
 *
 *         1) take each element into the first place, 
 *
 *             pos = 1
 *             [1, 2, 3]  ==>  [2, 1, 3] , [3, 1, 2] 
 *
 *             then we have total 3 answers
 *             [1, 2, 3],  [2, 1, 3] , [3, 1, 2] 
 *            
 *         2) take each element into the "first place" -- pos 
 *
 *             pos = 2
 *             [1, 2, 3]  ==>  [1, 3, 2]
 *             [2, 1, 3]  ==>  [2, 3, 1]
 *             [3, 1, 2]  ==>  [3, 2, 1] 
 *
 *             then we have total 6 answers
 *             [1, 2, 3],  [2, 1, 3] , [3, 1, 2], [1, 3, 2], [2, 3, 1], [3, 2, 1]
 *
 *          3) pos = 3 which greater than length of array, return.
 *
 */
vector<vector<int> > permute(vector<int> &num) {
    
    vector<vector<int> > vv;
    vv.push_back(num);

    if (num.size() <2){
        return vv;
    }
        
    int pos=0;
    while(pos<num.size()-1){
        int size = vv.size();
        for(int i=0; i<size; i++){
            //take each number to the first place
            for (int j=pos+1; j<vv[i].size(); j++) {
                vector<int> v = vv[i];
                int t = v[j]; 
                v[j] = v[pos];
                v[pos] = t;
                vv.push_back(v);
            }
        }
        pos++;
    }
    return vv;
}

int main(int argc, char** argv)
{
    int n = 3;
    if (argc>1){
       n = atoi(argv[1]); 
    }

    vector<int> v;
    for (int i=0; i<n; i++) {
        v.push_back(i+1);
    }
    vector<vector<int> > vv;
    vv = permute(v);
    
    for(int i=0; i<vv.size(); i++) {
        cout << "{ ";
        for(int j=0; j<vv[i].size(); j++){
            cout << vv[i][j] << " ";
        }
        cout << "}" <<endl;
    }

    return 0;
}

#include <algorithm>
#include <vector>

using std::vector; using std::next_permutation;

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > ret;
        sort(num.begin(), num.end());
        do {
            ret.push_back(num);
        } while (next_permutation(num.begin(), num.end()));
        return ret;
    }
};

/// Source : https://leetcode.com/problems/permutations/description/
/// Author : liuyubobobo
/// Time   : 2017-11-18

#include <iostream>
#include <vector>
using namespace std;

/// Most Naive Recursive
/// Time Complexity: O(n^n)
/// Space Complexity: O(n)
class Solution {

private:

    vector<vector<int>> res;
    vector<bool> used;

    void generatePermutation(const vector<int>& nums, int index, vector<int>& p){

        if(index == nums.size()){
            res.push_back(p);
            return;
        }

        for(int i = 0 ; i < nums.size() ; i ++)
            if(!used[i]){
                used[i] = true;
                p.push_back(nums[i]);
                generatePermutation(nums, index + 1, p );
                p.pop_back();
                used[i] = false;
            }

        return;
    }

public:
    vector<vector<int>> permute(vector<int>& nums) {

        res.clear();
        if(nums.size() == 0)
            return res;

        used = vector<bool>(nums.size(), false);
        vector<int> p;
        generatePermutation(nums, 0, p);

        return res;
    }
};


void printVec(const vector<int>& vec){
    for(int e: vec)
        cout << e << " ";
    cout << endl;
}

int main() {

    int nums[] = {1, 2, 3};
    vector<int> vec(nums, nums + sizeof(nums)/sizeof(int) );

    vector<vector<int>> res = Solution().permute(vec);
    for(int i = 0 ; i < res.size() ; i ++)
        printVec(res[i]);

    return 0;
}

/// Source : https://leetcode.com/problems/permutations/description/
/// Author : liuyubobobo
/// Time   : 2017-12-17

#include <iostream>
#include <vector>
using namespace std;

/// Recursive get all the permutations in place
/// Time Complexity: O(n!)
/// Space Complexity: O(n)
class Solution {

private:

    vector<vector<int>> res;

    void generatePermutation(vector<int>& nums, int index){

        if(index == nums.size()){
            res.push_back(nums);
            return;
        }

        for(int i = index ; i < nums.size() ; i ++){
            swap(nums[i], nums[index]);
            generatePermutation(nums, index + 1);
            swap(nums[i], nums[index]);
        }

        return;
    }

public:
    vector<vector<int>> permute(vector<int>& nums) {

        res.clear();
        if(nums.size() == 0)
            return res;

        generatePermutation(nums, 0);

        return res;
    }
};


void printVec(const vector<int>& vec){
    for(int e: vec)
        cout << e << " ";
    cout << endl;
}

int main() {

    vector<int> vec = {1, 2, 3};

    vector<vector<int>> res = Solution().permute(vec);
    for(int i = 0 ; i < res.size() ; i ++)
        printVec(res[i]);

    return 0;
}