LeetCode39 Combination Sum

题目：

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. （Medium）

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]
]

分析：

先排个序，用DFS搜索，每个数可选可不选，然后在start > end或者candidates[start] > target后就return。

恰好candidates[start] = target满足时添加到结果中。

代码：

 class Solution {
 private:
     vector<vector<int>>result;
     void dfs(int start, int end, const vector<int>& candidates, int target, vector<int>& internal) {
         if (start > end) {
             return;
         }
         if (candidates[start] == target) {
             internal.push_back(candidates[start]);
             result.push_back(internal);
             internal.pop_back();
             return;
         }
         if (candidates[start] > target) {
             return;
         }
         dfs(start + , end, candidates, target, internal);
         internal.push_back(candidates[start]);
         dfs(start, end, candidates, target - candidates[start], internal);
         internal.pop_back();
     }
 public:
     vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
         sort(candidates.begin(), candidates.end());
         int end = candidates.size() - ;
         vector<int> internal;
         dfs(, end, candidates, target, internal);
         return result;
     }
 };