算法设计与应用基础

216. 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.

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]]

只需要熟练运用向量就很容易完成

class Solution {
public:
  void combination(vector<vector<int>>& result, vector<int> sol, int k, int n) {
    if (sol.size() == k && n == 0) { result.push_back(sol); return ; }
    if (sol.size() < k) {
      for (int i = sol.empty() ? 1 : sol.back() + 1; i <= 9; ++i) {
        if (n - i < 0) break;
        sol.push_back(i);
        combination(result, sol, k, n - i);
        sol.pop_back();
      }
    }
  }


  vector<vector<int>> combinationSum3(int k, int n) {
    vector<vector<int>> result;
    vector<int> sol;
    combination(result, sol, k, n);
    return result;
  }
};