Combinations VS Permutation

Comination

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:
[

[1,2],

[1,3],

[1,4],

[2,4],
[2,3],
[3,4],
]

Solution

It's a way to simulate the process of getting 2 elements out of 4,

1. In round one, select 1 from 1...4 as the first number, you have [1]

2. next recursion, select one element from 2...n, you have [1,2], [1, 3], [1,4]

3 back to round one, select 2 from 2...4 as the first number, you have [2]

4. next recursion, select one element from 3..4, you have [2.3], [2,4]

......

	vector<vector<int> > combine(int n, int k) {
	vector<vector<int>> res;
	vector<int> sol;
	if(n<=0||k<=0||k>n){
	res.push_back(sol);
	return res;
	}
	combineRe(n,k,1,sol, res);
	return res;
	}
	void combineRe(int n, int k, int level, vector<int> &sol, vector<vector<int>> &res){
	if(sol.size() == k){
	res.push_back(sol);
	return;
	}
	for(int i=level; i<=n; i++){
	sol.push_back(i);
	combineRe(n, k, i+1, sol, res);
	sol.pop_back();
	}
	}

view raw gistfile1.cpp hosted with ❤ by GitHub

Permutation

Get all permutations of a string

Solution

Swap the first element with all the other elements and permute on the rest of the string

	void permutation(char[] s, int start, int end, vector<string> &res){
	if(start == end){
	string str(s);
	res.push_back(str);
	}
	else{
	for(int i= start; i<=end; i++){
	swap(s, start, i);
	permutation(s, start+1, end, res);
	swap(s, start, i);
	}
	}
	}

view raw gistfile1.cpp hosted with ❤ by GitHub