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],
]
Solution
经典 dfs + 回溯法
Complexity
时间复杂度 O(n!),空间复杂度 O(n)
Code
public class Solution {
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
ArrayList<Integer> path = new ArrayList<Integer>();
combineRe(n, k, 1, path, res);
return res;
}
void combineRe(int n, int k, int start, ArrayList<Integer> path, List<List<Integer>> res){
int m = path.size();
if (m == k) {
ArrayList<Integer> p = new ArrayList<Integer>(path);
res.add(p);
return;
}
for (int i = start; i <= n-(k-m)+1; ++i) {
path.add(i);
combineRe(n,k,i+1, path, res);
path.remove(path.size() - 1);
}
}
}