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.
All numbers will be positive integers
The solution set must not contain duplicate combinations.
Example
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]]
Note
组合从n个数中选k个数,n只能是[1, 9]闭区间的值,起始start为1,DFS循环遍历
当k和n都为0的时候,递归退出
注意剪枝:n小于0的情况或者在循环中n小于i
Code
class Solution {
public List<List<Integer>> combinationSum3(int k, int n) {
List<List<Integer>> res = new ArrayList<>();
helper(res, new ArrayList<>(), k, n, 1);
return res;
}
private static void helper(List<List<Integer>> res,
List<Integer> list, int k,
int n, int start) {
if (k == 0 && n == 0) {
res.add(new ArrayList<>(list));
return;
}
for (int i = start; i <= 9; i++) {
if (n < i) break;
list.add(i);
helper(res, list, k - 1, n - i, i + 1);
list.remove(list.size() - 1);
}
}
}
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