## BJP3 Exercise 17.14: completeToLevel

Author: Robert Baxter (on 2013/04/01)

Write a method `completeToLevel` that accepts an integer `n` as a parameter and that adds nodes to a tree so that the first `n` levels are complete. A level is complete if every possible node at that level is not `null`. We will use the convention that the overall root is at level `1`, it's children are at level `2`, and so on. You should preserve any existing nodes in the tree. Any new nodes added to the tree should have `-1` as their data.

For example, if a variable called `tree` refers to the tree below at left and you make the call of `tree.completeToLevel(3);`, the variable `tree` should store the tree below at right after the call.

Before Call After Call
```            +----+
| 17 |
+----+
/      \
/        \
+----+      +----+
| 83 |      |  6 |
+----+      +----+
/                  \
/                    \
+----+                  +----+
| 19 |                  | 87 |
+----+                  +----+
\                /
\              /
+----+      +----+
| 48 |      | 75 |
+----+      +----+
```
```                  +----+
| 17 |
_ +----+ _
_ /            \ _
/                    \
+----+                  +----+
| 83 |                  |  6 |
+----+                  +----+
/      \                /      \
/        \              /        \
+----+      +----+      +----+      +----+
| 19 |      | -1 |      | -1 |      | 87 |
+----+      +----+      +----+      +----+
\                            /
\                          /
+----+                  +----+
| 48 |                  | 75 |
+----+                  +----+
```

In this case, the request was to fill in nodes as necessary to ensure that the first `3` levels are complete. There were two nodes missing at level `3`, Notice that level `4` of this tree is not complete because the call requested that nodes be filled in to level `3` only.

Keep in mind that your method might need to fill in several different levels. Your method should throw an `IllegalArgumentException` if passed a value for level that is less than `1`.

Assume that you are adding this method to the `IntTree` class as defined below:

```public class IntTree {
private IntTreeNode overallRoot;
...
}
```
Type your solution here:

This is a partial class problem. Submit code that will become part of an existing Java class as described. You do not need to write the complete class, just the portion described in the problem.

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