LeetCode | Combination Sum

题目

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]

  分析

  用递归的动态规划解决，可以加上备忘录，如果target值允许的话，可以凶残的用非递归的数组方式解决。

  代码

  import java.util.ArrayList;
  import java.util.Arrays;
  
  public class CombinationSum {
  	public ArrayList
      
       > combinationSum(int[] candidates,
  			int target) {
  		Arrays.sort(candidates);
  		ArrayList
       
        > results = new ArrayList
        
         >(); ArrayList
         
           list = new ArrayList
          
           (); solve(results, list, candidates, 0, target); return results; } private void solve(ArrayList
           
            > results, ArrayList
            
              list, int[] candidates, int i, int target) { if (target < 0) { return; } if (target == 0) { results.add(new ArrayList
             
              (list)); return; } for (int j = i; j < candidates.length && candidates[j] <= target; ++j) { list.add(candidates[j]); solve(results, list, candidates, j, target - candidates[j]); list.remove(list.size() - 1); } } }