Java根据Integer数组（有null值）递归构造二叉树

二叉树：

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

递归构造二叉树：

public static TreeNode creatTreeNode(Integer[] arr, int index) {
    if (index >= arr.length) {
        return null;
    }

    if (arr[index] == null) {
        return null;
    }
    TreeNode root = new TreeNode(arr[index]);

    root.left = creatTreeNode(arr, ++index);
    root.right = creatTreeNode(arr, ++index);

    return root;
}