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_46.java
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_46.java
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package com.fishercoder.solutions;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class _46 {
public static class Solution1 {
public List<List<Integer>> permute(int[] nums) {
List<List<Integer>> result = new ArrayList();
result.add(new ArrayList<>());
return recursion(nums, 0, result);
}
private List<List<Integer>> recursion(int[] nums, int index, List<List<Integer>> result) {
if (index == nums.length) {
return result;
}
List<List<Integer>> newResult = new ArrayList<>();
for (List<Integer> eachList : result) {
for (int i = 0; i <= eachList.size(); i++) {
List<Integer> newList = new ArrayList<>(eachList);
newList.add(i, nums[index]);
newResult.add(newList);
}
}
result = newResult;
return recursion(nums, index + 1, result);
}
}
public static class Solution2 {
/**
* My completely original solution on 10/11/2021, although a little verbose and super as efficient as the above one.
* Basically, insert the next unused number into all possible positions in the currently formed list,
* as soon as the size of this list equals nums.length, add this new permutation into the result;
* I'll have to use a HashSet to filter out duplicates.
*/
public List<List<Integer>> permute(int[] nums) {
Set<List<Integer>> ans = new HashSet<>();
boolean[] used = new boolean[nums.length];
backtracking(new ArrayList<>(), nums, ans, used);
List<List<Integer>> result = new ArrayList<>();
for (List<Integer> list : ans) {
result.add(list);
}
return result;
}
private void backtracking(List<Integer> list, int[] nums, Set<List<Integer>> ans, boolean[] used) {
if (list.size() == nums.length) {
ans.add(new ArrayList<>(list));
return;
}
for (int i = 0; i <= list.size(); i++) {
for (int j = 0; j < nums.length; j++) {
if (!used[j]) {
used[j] = true;
list.add(i, nums[j]);
backtracking(list, nums, ans, used);
used[j] = false;
list.remove(i);
}
}
}
}
}
public static class Solution3 {
/**
* A more efficient version of backtracking.
*/
public List<List<Integer>> permute(int[] nums) {
List<List<Integer>> ans = new ArrayList<>();
boolean[] used = new boolean[nums.length];
return backtracking(ans, used, new ArrayList<>(), nums);
}
private List<List<Integer>> backtracking(List<List<Integer>> ans, boolean[] used, List<Integer> list, int[] nums) {
if (list.size() == nums.length) {
ans.add(new ArrayList<>(list));
return ans;
}
for (int i = 0; i < nums.length; i++) {
if (used[i]) {
continue;
}
used[i] = true;
list.add(nums[i]);
backtracking(ans, used, list, nums);
used[i] = false;
list.remove(list.size() - 1);
}
return ans;
}
}
}