LeetCode 216 : Combination Sum III

問題描述

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]]

java實作

回溯

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> result = new ArrayList<>();
        if (n < 1 || k <= 0)
            return result;
        backtrace(k, n, 1, new ArrayList<>(), result);
        return result;
    }

    private void backtrace(int k, int remain, int start, ArrayList<Integer> temp, List<List<Integer>> result) {
        if (k == 0 && remain == 0) {//結束條件
            result.add(new ArrayList<>(temp));
            return;
        }
        if (k == 0 || remain == 0)
            return;
        for (int i = start; i <= 9; i++) {
            temp.add(i);
            backtrace(k - 1, remain - i, i + 1, temp, result);
            temp.remove(temp.size() - 1);
        }
    }
}