问题描述
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实现
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) {//结束条件
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);
}
}
}