【Leetcode】【Medium】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]  

解题思路：

使用回溯的思想穷举可能的结果。

代码：

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> lst;
        vector<int> ans;
        sort(candidates.begin(), candidates.end());
        Backtracking(lst, ans, candidates, 0, target);
        return lst;
    }
    
    void Backtracking(vector<vector<int>> &lst, vector<int> ans, vector<int> candidates, int idx, int left) {
        for (int i = idx; i < candidates.size(); ++i) {
            int cur_left = left - candidates[i];
            if (cur_left == 0) {
                ans.push_back(candidates[i]);
                lst.push_back(ans);
                return;
            }
            if (cur_left > 0) {
                ans.push_back(candidates[i]);
                Backtracking(lst, ans, candidates, i, cur_left);
                ans.pop_back();
            } else {
                return;
            } 
        }
    }
};

另：是否还有DP/DFS等其他思路。