leetcode 216. Combination Sum III 求和III ---------- java

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]]

由于限定了1-9所以比较简单，枚举就可以。让答案数组升序排列，然后向里面放入数就可以了。

tips：用数组要比用ArrayList快。

public class Solution {
    public List<List<Integer>> list;
    public List<List<Integer>> combinationSum3(int k, int n) {
        list = new ArrayList();
        if (n > (18 - k + 1) * k / 2 || n < (1 + k) * k / 2 || k > 9){
            return list;
        }
        int[] nums = new int[k];
        for (int i = 1; i <= 10 - k; i++){
            nums[0] = i;
            getSum(nums, n, 0, k - 1);
        }
        return list;
    }
    public void getSum(int[] ans, int n, int pos, int k){
        if (k == 0){
            int sum = 0;
            for (int i = 0; i < ans.length; i++){
                sum += ans[i];
            }
            if (sum == n){
                List<Integer> ll = new ArrayList();
                for (int i : ans){
                    ll.add(i);
                }
                list.add(ll);
            }
            return ;
        }
        for (int i = ans[pos] + 1; i <= 10 - k; i++){
            ans[pos + 1] = i;
            getSum(ans, n, pos + 1, k - 1);
        }
    }
    
}