CombinationSum.java

package leetcode;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @author eko
 *
 * 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]
 * ]
 */
public class CombinationSum {
    public static void main(String[] args) {
        int[] candidates = new int[]{2,3,6,7};
        int target = 8;
        System.out.println(new CombinationSum().combinationSum(candidates, target));
    }

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        backtrack(res, new ArrayList<>(), candidates, target, 0);
        return res;
    }

    public void backtrack(List<List<Integer>> res, List<Integer> list, int[] candidates, int target, int start) {
        if (target < 0) return;
        if (target == 0) {
            res.add(new ArrayList<>(list));
            return;
        }

        for (int i = start; i < candidates.length; i++) {
            if (target >= candidates[i]) {
                list.add(candidates[i]);
                backtrack(res, list, candidates, target - candidates[i], i);
                list.remove(list.size()-1);
            }
        }
    }
}