leetcode 39 Combination Sum
z
 2018-03-04 22:04:14  

Given a set of candidate numbers (C) (without duplicates) 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
3
4
[
  [7],
  [2, 2, 3]
]
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2
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class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> ans = new ArrayList<List<Integer>>();
        if(candidates.length == 0) return ans;
        Arrays.sort(candidates);
        ArrayList<Integer> t = new ArrayList<Integer>();
        backtrack(candidates, target, ans, t, 0);
        return ans;
    }
    
    public void backtrack(int[] candidates, int target, List<List<Integer>> ans, ArrayList<Integer> t, int start){
        if(target == 0){
           ans.add(new ArrayList(t));
            return ;
        }
        if(target < 0)
            return ;
        
        for(int i=start; i<candidates.length; i++){
            if(candidates[i] <= target){
                t.add(candidates[i]);
                backtrack(candidates, target - candidates[i], ans, t, i);
                t.remove(t.size() - 1);
            }
        }

    }
}
  • 在不添加start的情况下，是直接的回溯法
  • 这里要考虑到需要取出重复操作。如果不加上start，会出现 [1,1,2] [2,1,1]同时出现的情况。