N个元素的所有子集问题（不可复选）

1.元素不可重复 

package com.hanzheng.algorithm.backtrace;

import java.util.LinkedList;
import java.util.List;

/**
 * 元素无重复，不可复选，N个元素的所有子集
 */
public class AllSubsets {
    static List<List<String>> res = new LinkedList<>();
    static LinkedList<String> track = new LinkedList<>();
    public static void main(String[] args) {
        String  [] nums = new String[]{"a","b"};
        res = subsets(nums);
        for (List<String> e : res) {
            System.out.println(e);
        }
    }

    private static List<List<String>> subsets(String[] nums) {
        backtrack(nums,0);
        return res;
    }

    private static void backtrack(String[] nums, int start) {
        // 到达一个节点就添加一个元素
        // []
        res.add(new LinkedList<>(track));
        for (int i = start; i < nums.length; i++) {
            // backtrack:[] --> [a]
            // backtrack:[] --> [a] --> [b]
            // backtrack:[] --> [a] --> [b] --> [c]
            track.add(nums[i]);
            backtrack(nums,i+1);
            track.removeLast();
        }
    }
}

2.元素可重复 

package com.hanzheng.algorithm.backtrace;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

public class AllSubsetsWithDuplicate {
    static List<List<String>> res = new LinkedList<>();
    static LinkedList<String> track = new LinkedList<>();

    public static void main(String[] args) {
        String[] nums = new String[]{"a", "b", "b","c"};
        res = subsets(nums);
        for (List<String> e : res) {
            System.out.println(e);
        }
    }

    private static List<List<String>> subsets(String[] nums) {
        Arrays.sort(nums);
        backtrack(nums, 0);
        return res;
    }

    private static void backtrack(String[] nums, int start) {
        res.add(new LinkedList<>(track));
        for (int i = start; i < nums.length; i++) {
            if (i > 0 && nums[i].equals(nums[i - 1])) {
                continue;
            }
            track.addLast(nums[i]);
            backtrack(nums, i + 1);
            track.removeLast();
        }

    }
}