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]

  題意：從數組中找到和為目標的組合，每個數允許重復。

  思路：排序後，利用回溯去做，為了讓每個數都能重復，所以枚舉的時候可以在從這個數的下標開始一次

  class Solution {
  public:
      void dfs(vector candidates, int index, int sum, int target, vector> &res, vector &path) {
          if (sum > target) return;
          if (sum == target) {
              res.push_back(path);
              return;
          }
          for (int i = index; i < candidates.size(); i++) {
              path.push_back(candidates[i]);
              dfs(candidates, i, sum+candidates[i], target, res, path);
              path.pop_back();
          }
      }
      vector > combinationSum(vector &candidates, int target) {
          sort(candidates.begin(), candidates.end());
          vector > res;
          vector path;
          dfs(candidates, 0, 0, target, res, path);
          return res;
      }
  };