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.
  • The solution set must not contain duplicate combinations.

For example, given candidate set [2, 3, 6, 7] and target 7,
A solution set is: 

  1. [
  2.   [7],
  3.   [2, 2, 3]
  4. ]

Solution:

  1. public class Solution {
  2.   public List<List<Integer>> combinationSum(int[] candidates, int target) {
  3.     Arrays.sort(candidates);
  4.     List<List<Integer>> res = new ArrayList<List<Integer>>();
  5.     helper(candidates, target, 0, new ArrayList<Integer>(), res);
  6.     return res;
  7.   }
  9.   private void helper(int[] candidates, int target, int index, List<Integer> sol, List<List<Integer>> res) {
  10.     if (target == 0) {
  11.       res.add(new ArrayList<Integer>(sol));
  12.       return;
  13.     }
  15.     if (target < 0 || index == candidates.length) {
  16.       return;
  17.     }
  19.     for (int i = index; i < candidates.length; i++) {
  20.       sol.add(candidates[i]);
  21.       helper(candidates, target - candidates[i], i, sol, res);
  22.       sol.remove(sol.size() - 1);
  23.     }
  24.   }
  25. }