As title said my solution works in place. However, depending on the concrete instance of the Triangle internal Lists the remove and add operations can take O(n) time. In such cases the time complexity is O(n^3).

```
public int minimumTotal(List<List<Integer>> triangle) {
if(triangle == null || triangle.size() == 0)
return 0;
List<Integer> currentRow, upperRow;
for(int i = triangle.size() - 1; i > 0; --i){
currentRow = triangle.get(i);
upperRow = triangle.get(i-1);
for(int j = 0; j < upperRow.size(); ++j){
int current = upperRow.remove(j);
upperRow.add(j , current + Math.min(currentRow.get(j),
currentRow.get(j+1)
)
);
}
}
return triangle.get(0).get(0);
}
```