[leetcode] Subsets II

Given a collection of integers that might contain duplicates, S, return all possible subsets.

Note:

Elements in a subset must be in non-descending order.

The solution set must not contain duplicate subsets.

https://oj.leetcode.com/problems/subsets-ii/

思路：关键是去重，先排序，然后根据后一个是否跟前一个相等来判断是否继续递归。

import java.util.ArrayList;
import java.util.Arrays;

public class Solution {
public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {

ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> tmp = new ArrayList<Integer>();
// sort first
Arrays.sort(num);
sub(num, 0, tmp, ans);
return ans;
}

public void sub(int[] num, int k, ArrayList<Integer> tmp, ArrayList<ArrayList<Integer>> ans) {
ArrayList<Integer> arr = new ArrayList<Integer>(tmp);
ans.add(arr);

for (int i = k; i < num.length; i++) {
// remove the dup
if (i != k && num[i] == num[i - 1])
continue;

tmp.add(num[i]);
sub(num, i + 1, tmp, ans);
tmp.remove(tmp.size() - 1);
}
}

public static void main(String[] args) {
System.out.println(new Solution().subsetsWithDup(new int[] { 1, 2, 2 }));
}
}

View Code

第二遍记录：

只用增量法实现了去重。

import java.util.ArrayList;
import java.util.Arrays;

public class Solution {
public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {

ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> tmp = new ArrayList<Integer>();
// sort first
Arrays.sort(num);
sub(num, 0, tmp, ans);
return ans;
}

public void sub(int[] num, int k, ArrayList<Integer> tmp, ArrayList<ArrayList<Integer>> ans) {
ArrayList<Integer> arr = new ArrayList<Integer>(tmp);
ans.add(arr);

for (int i = k; i < num.length; i++) {

if (i == k || num[i] != num[i - 1]) {
tmp.add(num[i]);
sub(num, i + 1, tmp, ans);
tmp.remove(tmp.size() - 1);
}
}
}

public static void main(String[] args) {
System.out.println(new Solution().subsetsWithDup(new int[] { 1, 2, 2 }));
}
}

cc150上也看到了，第三遍走起。

　　注意先排序。

　　注意递归条件中的start，表明可以开始选取元素的索引，因为我们每次只允许从后面选，递归向下传递start值为 i+1（不是start+1,开始这里搞错了）。

import java.util.ArrayList;

public class Solution {
public static ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
if (num == null || num.length == 0)
return res;
ArrayList<Integer> tmp = new ArrayList<Integer>();
subsetsHelper(num, res, tmp, 0);

return res;
}

private static void subsetsHelper(int[] num, ArrayList<ArrayList<Integer>> res, ArrayList<Integer> tmp, int start) {
res.add(new ArrayList<Integer>(tmp));

for (int i = start; i < num.length; i++) {

if (i == start || num[i] != num[i - 1]) {
tmp.add(num[i]);
subsetsHelper(num, res, tmp, i + 1);
tmp.remove(tmp.size() - 1);
}
}

}

public static void main(String[] args) {
int[] a = { 1, 2, 2 };

System.out.println(subsetsWithDup(a));
}
}

参考：

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