【LeetCode】Subsets 解题报告

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【题目】

Given a set of distinct integers, S, return all possible subsets.

Note:

Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.

For example,
If S = [1,2,3], a solution is:

[  [3],  [1],  [2],  [1,2,3],  [1,3],  [2,3],  [1,2],  []]

【解析】

题意：求一个集合的所有子集合，包括空集合和它本身。

与组合生成算法【LeetCode】Combinations 解题报告类似。

先排好序，从前往后取相应数目的元素。

【回溯法】

public class Solution {    int[] S = {};    int len = 0;    List<List<Integer>> ans = new ArrayList<List<Integer>>();        public List<List<Integer>> subsets(int[] S) {        this.S = S;        this.len = S.length;        Arrays.sort(S);                // length of subsets: from 0 to S.length        for (int i = 0; i <= len; i++) {            backTracking(new ArrayList<Integer>(), 0, i);        }        return ans;    }        public void backTracking(List<Integer> list, int from, int tar) {        if (list.size() == tar) {            List<Integer> res = new ArrayList<Integer>(list);            ans.add(res);        } else {            for (int i = from; i < len; i++) {                list.add(S[i]);                backTracking(list, i + 1, tar);                list.remove(new Integer(S[i]));            }        }    }}

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