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LC_77_combinations.java
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37 lines (33 loc) · 1.09 KB
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package Week03;
import java.util.ArrayList;
import java.util.List;
public class LC_77_combinations {
//iterrate 1..n
//recursive
//every layer, need a loop to get rest of value for n, until reach k.
//在每一层中，把余下的值加到list中直到k,然后输出list.
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> ans = new ArrayList<>();
if (k == 0 ){
return ans;
}
List<Integer> list = new ArrayList<>();
dps(ans, list, 1, k ,n);
return ans;
}
private void dps(List<List<Integer>> ans, List<Integer> list, int begin, int k, int n){
if (list.size() == k){
//need new a list
//here is the output of stack
ans.add(new ArrayList<>(list));
return;
}
for (int i = begin;i <= n;i++) {
list.add(i);
//add 1 to Stack, then 2, then 3 .. all n add to stack.
//then go to for loop. add second number.
dps(ans,list, i+1,k,n);
list.remove(list.size()-1);
}
}
}