[LeetCode]--39. Combination Sum

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:
All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
For example, given candidate set [2, 3, 6, 7] and target 7,
A solution set is:

[
  [7],
  [2, 2, 3]
]

我想好半天也没把递归搞清楚，在网上看了一个比较好的算法，跟着学了一遍，debug跟着递归走一走，感觉拨开迷雾一样。

public List<List<Integer>> combinationSum(