[LeetCode19]Combinations

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:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Analysis:

DFS problem. similar with combination sum

Java

public class Solution {
    List<List<Integer>> result;
	List<Integer> solu;
	public List<List<Integer>> combine(int n, int k) {
        result = new ArrayList<>();
        solu = new ArrayList<>();
        getCombine(n, k, 1);
        return result;
    }
	public void getCombine(int n, int k, int st){
		if(solu.size() == k){
			result.add(new ArrayList<>(solu));
			return;
		}
		for(int i=st;i<=n;i++){
			solu.add(i);
			getCombine(n, k,i+1);
			solu.remove(solu.size()-1);
		}
	}
}

c++

class Solution {
public:
    void getCombine(
                int n,
                int k,
                int level,
                vector<int> &solution,
                vector<vector<int>> &result){
    if(solution.size() == k){
        result.push_back(solution);
        return;
    }
    for(int i = level; i<=n; i++){
        solution.push_back(i);
        getCombine(n,k,i+1,solution,result);
        solution.pop_back();
    }
}
vector<vector<int> > combine(int n, int k) {
    vector<int> solution;
    vector<vector<int>> result;
    getCombine(n,k,1,solution,result);
    return result;
}
};