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 + 1, 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() - 1;
        vector<int> internal;
        dfs(0, end, candidates, target, internal);
        return result;
    }
};