This is the question:https://leetcode.com/problems/combinations/
This is my solution 1:
public class Solution {
public List<List<Integer>> combine(int n, int k){
List<List<Integer>> result = new ArrayList<List<Integer>>();
combine(n, k, 1, result, new ArrayList<Integer>());
return result;
}
public void combine(int n, int k , int start, List<List<Integer>> result, ArrayList<Integer> l){
if(k == 0){
result.add(l);
return;
}
for(int i = start; i <= n; ++i){
l.add(i);
combine(n, k - 1, i + 1, result, l);
}
}
}
Result: small test cases passed. But big test cases time exceed.
Submission Result: Time Limit Exceeded Last executed input: 10, 5
Solution 2:
public class Solution {
public List<List<Integer>> combine(int n, int k){
List<List<Integer>> result = new ArrayList<List<Integer>>();
combine(n, k, 1, result, new ArrayList<Integer>());
return result;
}
public void combine(int n, int k , int start, List<List<Integer>> result, ArrayList<Integer> l){
if(k == 0){
result.add(l);
return;
}
for(int i = start; i <= n; ++i){
ArrayList<Integer> a = (ArrayList<Integer>) l.clone();
a.add(i);
combine(n, k - 1, i + 1, result, a);
}
}
}
Passed all test cases.
the main difference is clone of the list. But why? is the solution A wrong or just slow? Why using clone is faster here? Really confused.
