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],
]
解题思路：
DFS，JAVA实现如下：
public static void main(String args[]) {
List<List<Integer>> list = combine(5, 2);
System.out.println(list.size());
for (List<Integer> alist : list)
System.out.println(alist + "*");
}
static public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> list = new ArrayList<List<Integer>>();
if (n <= 0 || k > n)
return list;
dfs(list, n, k, 0);
return list;
}
static List<Integer> alist = new ArrayList<Integer>();
static void dfs(List<List<Integer>> list, int n, int k, int depth) {
if (depth >= k) {
list.add(new ArrayList<Integer>(alist));
return;
}
if (depth == 0)
for (int i = 1; i <= n - k + 1; i++) {
alist.add(i);
dfs(list, n, k, depth + 1);
alist.remove(alist.size() - 1);
}
else
for (int i = 1; i <= n - alist.get(alist.size() - 1) + k - depth - 1; i++) {
alist.add(alist.get(alist.size() - 1) + i);
dfs(list, n, k, depth + 1);
alist.remove(alist.size() - 1);
}
}