Combination Sum III

 
 
 Combination Sum III
 
 
Medium
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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.
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.
Example 3:

Input: k = 4, n = 1
Output: []
Explanation: There are no valid combinations.
Using 4 different numbers in the range [1,9], the smallest sum we can get is 1+2+3+4 = 10 and since 10 > 1, there are no valid combination.
 

Constraints:

2 <= k <= 9
1 <= n <= 60



code:-  
class Solution:
    def combinationSum3(self, k: int, n: int) -> List[List[int]]:
        op=[]
        def comb(k,n,i,op,tem,last):
            if i==k:
                if n==0:
                    op.append(tem.copy())
                return
            for j in range(last+1,10):
                if j<=n:
                    tem.append(j)
                    comb(k,n-j,i+1,op,tem,j)
                    tem.pop(-1)
        comb(k,n,0,op,[],0)  
        return op