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LC_78_subsets.java
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60 lines (53 loc) · 1.79 KB
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package Week03;
import java.util.ArrayList;
import java.util.List;
public class LC_78_subsets {
/* //Solutions:
* 1. Recusive
* dps: for any element, there has only two options:
* select or no-select. in every level save the result to output
* as we change the param, need to reverse the changes
* */
public List<List<Integer>> subsets(int[] nums) {
List<List<Integer>> ans = new ArrayList<>();
if (nums.length == 0 ){
return ans;
}
List<Integer> list = new ArrayList<>();
dfs(ans,nums,list,0);
return ans;
}
private void dfs(List<List<Integer>> ans, int[] nums, List<Integer> list, int level) {
//走到的数组nums的末尾了
if (level == nums.length){
//走到的数组nums的末尾了，把结果放入answer
ans.add(list);
return;
}
dfs(ans,nums,list,level+1);
list.add(nums[level]);
dfs(ans,nums,list,level+1);
list.remove(list.size()-1);
}
/*
* 2. Iterative
* 逐个枚举，空集的幂集只有空集，每增加一个元素，让之前幂集中的每个集合，追加这个元素，就是新增的子集。
*
* */
public List<List<Integer>> subsets_iterate(int[] nums) {
List<List<Integer>> ans = new ArrayList<>();
//加一个空集[[]]
ans.add(new ArrayList<Integer>());
for (int n: nums) {
int size = ans.size();
for (int i =0; i<size; i++ ){
//把以前的集取出，做成一个新集
List<Integer> newList = new ArrayList<>(ans.get(i));
//加入新的元素
newList.add(n);
ans.add(newList);
}
}
return ans;
}
}