从上往下打印二叉树（分层遍历）

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
};

void LevelOrder(BinaryTreeNode* root) {
    if (root == NULL) return;

    queue<BinaryTreeNode*> q;

    q.push(root);

    while (!q.empty()) {
        BinaryTreeNode* p = q.front();
        q.pop();

        cout<<p->val<<" ";

        if (p->left)
            q.push(p->left);

        if (p->right)
            q.push(p->right);
    }
}