216. Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

思路：

使用递归，循环尝试每一种可能

public class Solution {    private static List<List<Integer>> res;    public List<List<Integer>> combinationSum3(int k, int n) {res = new ArrayList<List<Integer>>();if (k < 1 || n < 1)return res;List<Integer> solution = new ArrayList<Integer>();findSolution(k, n, 0, solution, 1);return res;    }    private static void findSolution(int k, int n, int sum,List<Integer> solution, int pre) {if (sum == n && k == 0) {res.add(new ArrayList<Integer>(solution));} else if (sum > n || k <= 0) {return;}for (int i = pre; i <= 9; i++) {solution.add(i);findSolution(k - 1, n, sum + i, solution, i + 1);solution.remove(solution.size() - 1);}}}