216. Combination Sum III - jiejackyzhang/leetcode-note GitHub Wiki

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]]

解题思路为backtracking。

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