【LeetCode】216. Combination Sum III

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.

Ensure that numbers within the set are sorted in ascending order.

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]]

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

参考Combination Sum II，把去重条件去掉（1-9本身就不包含重复元素），仅返回包含k个元素的vector

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<int> num(9);
        for(int i = 0; i < 9; i ++)
            num[i] = i+1;
        return combinationSum2(num, n, k);
    }
    vector<vector<int> > combinationSum2(vector<int> &num, int target, int k) {
        sort(num.begin(), num.end());
        vector<vector<int> > ret;
        vector<int> cur;
        Helper(ret, cur, num, target, 0, k);
        return ret;
    }
    void Helper(vector<vector<int> > &ret, vector<int> cur, vector<int> &num, int target, int position, int k)
    {
        if(target == 0)
        {
            if(cur.size() == k)
                ret.push_back(cur);
            else
                ;
        }
        else
        {
            for(int i = position; i < num.size() && num[i] <= target; i ++)
            {
                cur.push_back(num[i]);
                Helper(ret, cur, num, target-num[i], i+1, k);
                cur.pop_back();
            }
        }
    }
};