Created
October 20, 2022 14:37
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Permutation
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// https://leetcode.com/problems/permutations/ | |
class Solution { | |
public List<List<Integer>> permute(int[] nums) { | |
List<List<Integer>> ans = new ArrayList(); | |
if(nums == null || nums.length == 0) return ans; | |
ans.add(new ArrayList()); | |
for(int i = 0; i < nums.length; i++) { | |
int size = ans.size(); | |
int num = nums[i]; | |
for(int j = 0 ; j < size; j++) { | |
// poll first from ans | |
List<Integer> list = ans.get(0); | |
// loop through it | |
for(int k = 0; k <= list.size(); k++) { | |
// clone it | |
List<Integer> clonedList = new ArrayList(list); | |
if(k < list.size()) { | |
// add num in each one of them in each index | |
clonedList.add(k, num); | |
} else { | |
// add num in each one of them in each index (at to last) | |
clonedList.add(num); | |
} | |
// add cloned list | |
ans.add(clonedList); | |
} | |
// remove the original list | |
ans.remove(0); | |
} | |
} | |
return ans; | |
} | |
} | |
// 1,2,3 | |
// pre-add empty list | |
// [] | |
// poll lists from ans | |
// clone it | |
// add num in each one of them in each index | |
// remove the original list | |
// [1] | |
// poll lists from ans | |
// clone it | |
// add num in each one of them in each index | |
// remove the original list | |
// [1,2] [2, 1] | |
// poll lists from ans | |
// clone it | |
// add num in each one of them in each index | |
// remove the original list | |
// [3,1,2] [3, 2, 1] [1,3, 2] [2,3, 1] [1,2, 3] [2, 1, 3] |
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Outer loop = O(n)
2 inner loops = O(n!) since it is a permutation. E.g. [1,2,3,4] has 24 permutations [123*4]
Time = O(n * n!)