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, … , a_k) must be in non-descending order. (ie, a₁ ≤ a₂ ≤ … ≤ a_k).
• 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]

 

 1 public class Solution {

 2     ArrayList<ArrayList<Integer>> result = null;

 3     public ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {

 4         // Note: The Solution object is instantiated only once and is reused by each test case.

 5         result = new ArrayList<ArrayList<Integer>>();

 6         getSolution(new ArrayList<Integer>(), target, candidates);

 7         return result;

 8     }

 9     public void getSolution(ArrayList<Integer> row, int target, int[] candidates){

10         int i = 0;

11         if(target == 0){

12             result.add(row);

13         }

14         while(i < candidates.length && candidates[i] <= target){

15             row.add(candidates[i]);

16             target -= candidates[i];

17             getSolution(new ArrayList<Integer>(row),target,candidates);

18             row.remove(row.size() - 1);

19             target += candidates[i];

20             i ++;

21         }

22         if(i == 0){

23             return;

24         }

25     }

26 }

没有去重。

eg: [1,1,2] 3

去重包括两部分，1.array 里面有重复： 1，1

                     2.顺序， 如 1,2 ; 2, 1

对于1，先将数组去重。

对于2，后面的选择范围只能从当前选择的元素开始，只能选不比它小的元素。

 1 public class Solution {

 2     ArrayList<ArrayList<Integer>> result = null;

 3     public ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {

 4         // Note: The Solution object is instantiated only once and is reused by each test case.

 5         Arrays.sort(candidates);

 6         int start = 0;

 7         for(int i = 1; i < candidates.length; i ++){

 8             if(candidates[i] != candidates[start]){

 9                 candidates[++start] = candidates[i];

10             }

11         }

12         result = new ArrayList<ArrayList<Integer>>();

13         getSolution(new ArrayList<Integer>(), target, candidates, start, 0);

14         return result;

15     }

16     public void getSolution(ArrayList<Integer> row, int target, int[] candidates, int len, int pos){

17         int i = pos;

18         if(target == 0){

19             result.add(row);

20         }

21         while(i <= len && candidates[i] <= target){

22             row.add(candidates[i]);

23             target -= candidates[i];

24             getSolution(new ArrayList<Integer>(row), target, candidates, len, i);

25             row.remove(row.size() - 1);

26             target += candidates[i];

27             i ++;

28         }
29     }

30 }

 第三遍：

 1 public class Solution {

 2     public List<List<Integer>> combinationSum(int[] candidates, int target) {

 3         int len = 0;

 4         Arrays.sort(candidates);

 5         for(int i = 0; i < candidates.length; i ++){

 6             if(i == 0 || candidates[i] != candidates[i - 1]){

 7                 candidates[len ++] = candidates[i];

 8             }

 9         }

10         List<List<Integer>> result = new ArrayList<List<Integer>>();

11         findElement(candidates, 0, len, result, new ArrayList<Integer>(), target);

12         return result;

13     }

14     

15     public void findElement(int[] arr, int start, int end, List<List<Integer>> result, ArrayList<Integer> row, int tar){

16         if(tar == 0) result.add(row);

17         for(int i = start; i < end; i ++){

18             if(arr[i] <= tar){

19                 ArrayList<Integer> rown = new ArrayList<Integer> (row);

20                 rown.add(arr[i]);

21                 findElement(arr, i, end, result, rown, tar - arr[i]);

22             }

23         }

24     }

25 }