LeetCode216. Combination Sum III

LeetCode216. Combination Sum III

題目：

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]]

題意分析：

這道題和LeetCode39基本用的是同一個思路。也是揹包問題的擴展版。屬於回溯的經典問題。

void isok(vector<vector<int>>& result, int n, int begin, vector<int>& tmp, int k) { //n:target
	if (n == 0 && tmp.size() == k) {
		result.push_back(tmp);
		return;
	}
	else {
		for (int i = begin; i <= 9; i++) {
			if (i > n) break;
			tmp.push_back(i);
			isok(result, n - i, i+1, tmp, k);
			tmp.pop_back();
		}
	}
}

代碼：

class Solution {
public:
	vector<vector<int>> combinationSum3(int k, int n) {
		vector<vector<int>> result;
		vector<int> tmp;
		isok(result, n, 1, tmp, k);
		return result;
	}

	void isok(vector<vector<int>>& result, int n, int begin, vector<int>& tmp, int k) { //n:target
		if (n == 0 && tmp.size() == k) {
			result.push_back(tmp);
			return;
		}
		else {
			for (int i = begin; i <= 9; i++) {
				if (i > n) break;
				tmp.push_back(i);
				isok(result, n - i, i+1, tmp, k);
				tmp.pop_back();
			}
		}
	}
};