Combination Sum

Given a set of candidate numbers ( candidates ) (without duplicates) and a target number ( target ), find all unique combinations in candidates where the candidate numbers sums to target .
The same repeated number may be chosen from candidates unlimited number of times.    [#39]
Note:
All numbers (including target ) will be positive integers.
The solution set must not contain duplicate combinations.

Example 1:
Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
[7],
[2,2,3]
]

Example 2:
Input: candidates = [2,3,5], target = 8,
A solution set is:
[
[2,2,2,2],
[2,3,3],
[3,5]
]

Combination Sum II

Given a collection of candidate numbers ( candidates ) and a target number ( target ), find all unique combinations in candidates where the candidate numbers sums to target .
Each number in candidates may only be used once in the combination.       [#40]
Note:
All numbers (including target ) will be positive integers.
The solution set must not contain duplicate combinations.

Example 1:
Input: candidates = [10,1,2,7,6,1,5], target = 8,
A solution set is:
[
[1, 7],
[1, 2, 5],
[2, 6],
[1, 1, 6]
]

Example 2:
Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
[1,2,2],
[5]
]

Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.    [#216]
Note:
All numbers will be positive integers.
The solution set must not contain duplicate combinations.

Example 1:
Input: k = 3, n = 7
Output: [[1,2,4]]

Example 2:
Input: k = 3, n = 9
Output: [[1,2,6], [1,3,5], [2,3,4]]