leetcode — 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.
– Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
– 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]

思路

T=a1+a2+...+ak，首先固定某1个元素ai(1≤i≤k),则剩余元素相加等于T?ai.而剩余元素相加可以以下递归vector<vector<int> > pre = combinationSum(c, target - *i)。其中的1个目的是为了快速减小target（以致target=a1乃至target<a1