39. Combination Sum

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

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

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]

Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

思路

该问题不穷举一把没法知道所有符合要求的答案，所以符合使用回溯法来求解的条件。

C++11

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> res;
        std::sort(candidates.begin(), candidates.end());
        dfs(candidates, target, vector<int>{}, 0, res);
        return res;
    }

    void dfs(const vector<int> &candidates, int target, vector<int> curr, size_t index, vector<vector<int>> &res) {
        if (target == 0) {
            res.emplace_back(curr);
            return;
        }
        for (auto i = index; i < candidates.size(); ++i) {
            if (candidates[i] <= target) {
                curr.emplace_back(candidates[i]);
                dfs(candidates, target - candidates[i], curr, i, res);
                curr.pop_back();
            } else break; 
        }
    }
};