// typedef struct {
// int value;
// struct TreeNode *left;
// struct TreeNode *right;
// } TreeNode;
-(NSMutableArray *)preorderTraversal:(TreeNode *)treeNode
{
NSMutableArray *preorderArray = [NSMutableArray array];
if (!treeNode) {
[array addObject: treeNode];
preorderTraversal(treeNode.left);
preorderTraversal(treeNode.right);
}
return preorderArray;
}