 Time Flying  ### 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 = 7Output: [[1,2,4]]
Explanation:1 + 2 + 4 = 7There are no other valid combinations.

Example 2:

Input: k = 3, n = 9Output: [[1,2,6],[1,3,5],[2,3,4]]
Explanation:1 + 2 + 6 = 91 + 3 + 5 = 92 + 3 + 4 = 9There are no other valid combinations.

Example 3:

Input: k = 4, n = 1Output: []
Explanation: There are no valid combinations. [1,2,1] is not valid because 1 is used twice.

Example 4:

Input: k = 3, n = 2Output: []
Explanation: There are no valid combinations.

Example 5:

Input: k = 9, n = 45Output: [[1,2,3,4,5,6,7,8,9]]
Explanation:1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45​​​​​​​There are no other valid combinations.

Constraints:

• 2 <= k <= 9
• 1 <= n <= 60

### Analyze

/** * @param {number} k * @param {number} n * @return {number[][]} */var combinationSum3 = function(k, n) {  const result = []
recursive(k, n, 1, [], result)  return result};
var recursive = function(k, n, start, temp, result) {  if (n < 0 || temp.length > k) return  if (n === 0 && temp.length === k) {    result.push([...temp])    return  }
for (let i = start; i <= 9; i++) {    temp.push(i)    n = n - i    recursive(k, n, i + 1, temp, result)    n = n + i    temp.pop()  }}

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