中序遍历

void bfs(TreeNode* root){
    if (!root)return;
    stack<TreeNode*> s;
    TreeNode *p = root;
    while (!s.empty() || p){
        while (p){
            s.push(p);
            p = p->left;
        }
        p = s.top();
        cout << p->val << endl;//中序遍历
        s.pop();
        p = p->right;
    }
}