216. Combination Sum III

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]]

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

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