Combination Sum 3
Application of Backtracking

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Problem Statement:
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. [1,2,1] is not valid because 1 is used twice.
Example 4:
Input: k = 3, n = 2
Output: []
Explanation: There are no valid combinations.
Example 5:
Input: k = 9, n = 45
Output: [[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.
Solution:
 NOTE: I highly recommend going through the Backtracking chapters in the order they are given in the Index page to get the most out of it and be able to build a rocksolid understanding.
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Don't forget to take indepth look at the other backtracking problems because that is what would make you comfortable with using the backtracking template and master the art of Backtracking:
 Letter Case Permutation
 Power Set
 All Paths Between Two Nodes
 Word Search
 Sudoku
 NQueens
 Word Square
 Generate Parentheses
Instructor:
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