题目来源
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]]
【Combination|Combination Sum III】Example 2:
Input: k = 3, n = 9
Output:
[[1,2,6], [1,3,5], [2,3,4]]
这道题的想法，就是直接递归回溯实现。
class Solution {
public:
vector> combinationSum3(int k, int n) {
vector> res;
if (n > 45)
return res;
vector item;
cal(res, item, k, n, 1);
return res;
}void cal(vector> &res, vector &item, int k, int n, int i)
{
if (k == 0 && n == 0) {
res.push_back(item);
return;
}
if (i > 9)
return;
item.push_back(i);
cal(res, item, k-1, n-i, i+1);
item.pop_back();
cal(res, item, k, n, i+1);
}
};
