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454. Find all unique combinations of numbers (from 1 to 9 ) with sum to N
Objective: Given an integer N, Write an algorithm to select numbers from 1 to 9 and make combinations such that its sum to N.
Example:
N= 5 Output: [1, 4] [2, 3] [5] N =12 [1, 2, 3, 6] [1, 2, 4, 5] [1, 2, 9] [1, 3, 8] [1, 4, 7] [1, 5, 6] [2, 3, 7] [2, 4, 6] [3, 4, 5] [3, 9] [4, 8] [5, 7]
Approach: Use Recursion
 Given N.
 Start with sum = 0, start = 0, combinationList=null
 Iterate through i = start to 9.
 Add i to sum.
 Put i to combinationList.
 Make a recursive call with start = start + 1 (to avoid duplicates) and combinationList.
 In tail recursion, backtrack and remove i from the combinationList to find more solutions
 Base case: if sum=N, Print combinationList.
Output:
N = 12 [1, 2, 3, 6] [1, 2, 4, 5] [1, 2, 9] [1, 3, 8] [1, 4, 7] [1, 5, 6] [2, 3, 7] [2, 4, 6] [3, 4, 5] [3, 9] [4, 8] [5, 7]