【leetcode】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.

Note:

• All numbers will be positive integers.
• The solution set must not contain duplicate combinations.

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]]

 

題解如下：

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> tmp = new ArrayList<>();
        backtrack(res,tmp,k,n,0);
        return res;
    }
    
    public void backtrack(List<List<Integer>> res,List<Integer> tmp,
                         int k,int n,int idx) {
        if(k == 0 && n == 0) {
            res.add(new ArrayList(tmp));
            return;
        }
        for(int i = idx;i < 9;i++) {
            int num = i + 1;
            if(k > 0 && n >= num) {
                tmp.add(num);
                backtrack(res,tmp,k-1,n-num,i+1);
                tmp.remove(tmp.size()-1);
            }
        }
    }
}