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427\. Construct Quad Tree

Medium

Given a `n * n` matrix `grid` of `0's` and `1's` only. We want to represent the `grid` with a Quad-Tree.

Return _the root of the Quad-Tree_ representing the `grid`.

Notice that you can assign the value of a node to **True** or **False** when `isLeaf` is **False**, and both are **accepted** in the answer.

A Quad-Tree is a tree data structure in which each internal node has exactly four children. Besides, each node has two attributes:

*   `val`: True if the node represents a grid of 1's or False if the node represents a grid of 0's.
*   `isLeaf`: True if the node is leaf node on the tree or False if the node has the four children.
```
    class Node {
        public boolean val;
        public boolean isLeaf;
        public Node topLeft;
        public Node topRight;
        public Node bottomLeft;
        public Node bottomRight;
    }
```
We can construct a Quad-Tree from a two-dimensional area using the following steps:

1.  If the current grid has the same value (i.e all `1's` or all `0's`) set `isLeaf` True and set `val` to the value of the grid and set the four children to Null and stop.
2.  If the current grid has different values, set `isLeaf` to False and set `val` to any value and divide the current grid into four sub-grids as shown in the photo.
3.  Recurse for each of the children with the proper sub-grid.

![](https://assets.leetcode.com/uploads/2020/02/11/new_top.png)

If you want to know more about the Quad-Tree, you can refer to the [wiki](https://en.wikipedia.org/wiki/Quadtree).

**Quad-Tree format:**

The output represents the serialized format of a Quad-Tree using level order traversal, where `null` signifies a path terminator where no node exists below.

It is very similar to the serialization of the binary tree. The only difference is that the node is represented as a list `[isLeaf, val]`.

If the value of `isLeaf` or `val` is True we represent it as **1** in the list `[isLeaf, val]` and if the value of `isLeaf` or `val` is False we represent it as **0**.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/02/11/grid1.png)

**Input:** grid = [[0,1],[1,0]]

**Output:** [[0,1],[1,0],[1,1],[1,1],[1,0]]

**Explanation:**

    The explanation of this example is shown below:
    Notice that 0 represnts False and 1 represents True in the photo representing the Quad-Tree.

![](https://assets.leetcode.com/uploads/2020/02/12/e1tree.png) 

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/02/12/e2mat.png)

**Input:** grid = [[1,1,1,1,0,0,0,0],[1,1,1,1,0,0,0,0],[1,1,1,1,1,1,1,1],[1,1,1,1,1,1,1,1],[1,1,1,1,0,0,0,0],[1,1,1,1,0,0,0,0],[1,1,1,1,0,0,0,0],[1,1,1,1,0,0,0,0]]

**Output:** [[0,1],[1,1],[0,1],[1,1],[1,0],null,null,null,null,[1,0],[1,0],[1,1],[1,1]]

**Explanation:**

    All values in the grid are not the same. We divide the grid into four sub-grids.
    The topLeft, bottomLeft and bottomRight each has the same value.
    The topRight have different values so we divide it into 4 sub-grids where each has the same value.
    Explanation is shown in the photo below:
    
![](https://assets.leetcode.com/uploads/2020/02/12/e2tree.png) 

**Constraints:**

*   `n == grid.length == grid[i].length`
*   n == 2^x where `0 <= x <= 6`