Solution 21: 子集和问题

public class SubsetSum {
public List<List<Integer>> getSubset(int n, int m) {
List<List<Integer>> res = new ArrayList<List<Integer>>();

if (n <= 0 || m <= 0 || m > n * (n + 1) / 2) {
return res;
}

int[] nums = new int
;
for (int i = 0; i < nums.length; i++) {
nums[i] = i + 1;
}

List<Integer> sol = new ArrayList<Integer>();
helper(res, sol, nums, m, 0);
return res;
}

private void helper(List<List<Integer>> res, List<Integer> sol, int[] nums, int m, int start) {
if (m < 0) {
return ;
}
if (m == 0) {
res.add(new ArrayList<Integer>(sol));
return;
}

for (int i = start; i < nums.length; i++) {
if (sol.contains(nums[i])) {
continue;
}
sol.add(nums[i]);
helper(res, sol, nums, m - nums[i], i + 1);
sol.remove(sol.size() - 1);
}
}
}