Combination Sum III backtracking
Description
216. Combination Sum III
Find all valid combinations of k numbers that sum up to n such that the following conditions are true:
Only numbers 1 through 9 are used. Each number is used at most once. Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.
Example 1:
Input: k = 3, n = 7
Output: [[1,2,4]]
Explanation:
1 + 2 + 4 = 7
There are no other valid combinations.
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Example 2:
Input: k = 3, n = 9
Output: [[1,2,6],[1,3,5],[2,3,4]]
Explanation:
1 + 2 + 6 = 9
1 + 3 + 5 = 9
2 + 3 + 4 = 9
There are no other valid combinations.
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Constraints:
2 <= k <= 9
1 <= n <= 60
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Solution
/*
* @lc app=leetcode.cn id=216 lang=cpp
*
* [216] 组合总和 III
*/
#include <vector>
using namespace std;
// @lc code=start
class Solution
{
public:
vector<vector<int>> res;
vector<int> path;
void backtracking(int startIndex, int sum, int k, int n)
{
if (path.size() == k)
{
//check if it's equal to the target int n
if (sum == n)
{
//if the sum equals to int n,
//we save it into the vector res
res.push_back(path);
}
return;
}
//(k-path.size()) means still need how many elements to reach total amount of k
for (int i = startIndex; i <= 9 - (k - path.size()) + 1; i++)
{
sum += i;
path.push_back(i);
backtracking(i + 1, sum, k, n);
sum -= i;
path.pop_back();
}
}
vector<vector<int>> combinationSum3(int k, int n)
{
backtracking(1, 0, k, n);
return res;
}
};
// @lc code=end
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