[LeetCode 216] Combination Sum III

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

Ensure that numbers within the set are sorted in ascending order.

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]]

Solution:

DFS, two conditions for basic case, one is sum , second is k number

public List<List<Integer>> combinationSum3(int k, int n) {List<List<Integer>> result = new ArrayList<>();List<Integer> cur = new ArrayList<>();if(k<1 || n<1 || k>n) return result;dfs(result, cur, 0, k, n, 1);return result;}public void dfs(List<List<Integer>> result, List<Integer> cur, int sum, int k, int n, int level) {if(sum == n && k==0) {result.add(new ArrayList(cur));return;} else if(sum> n || k<0) return;for(int i=level;i<=9;i++){cur.add(i);dfs(result, cur, sum+i, k-1, n, i+1);cur.remove(cur.size()-1);}}

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