40. Combination Sum II

Posted 2021-02-12 wentiliangkaihua

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

Each number in candidates may only be used once in the combination.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [10,1,2,7,6,1,5], target = 8,
A solution set is:
[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]

Example 2:

Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
  [1,2,2],
  [5]
]

 

class Solution {
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        if(candidates.length==0) return new ArrayList();
        Arrays.sort(candidates);
        List<List<Integer>> results = new ArrayList();
        List<Integer> result = new ArrayList();
        dfs(candidates, results, result, target, 0);
        return results;
    }
    public void dfs(int[] candidates,List<List<Integer>> results,List<Integer> result, int target, int start ){
        if(target==0){
            System.out.println(result);
            results.add(new ArrayList(result));
            return;
        }
        int pre = -1;

        for(int i = start; i < candidates.length; i++){
            if(pre==candidates[i]) continue;//用previous防止重复
            if((target-candidates[i]) < 0){
                return;
            }
            pre = candidates[i];
            result.add(candidates[i]);
            dfs(candidates, results, result, target-candidates[i],i+1);//i变成i+1
            result.remove(result.size()-1);
        }
    }
}

     // 如果上一轮循环已经使用了nums[i]，则本次循环就不能再选nums[i]，
            // 确保nums[i]最多只用一次