//stack

vector<int> inorderTraversal(TreeNode* root) {

stack<TreeNode*> s;

vector<int> ret;

TreeNode *p=root;

while(p||!s.empty()){

if(p){

s.push(p);

p=p->left;

}else{

p=s.top();

s.pop();

ret.push_back(p->val);

p=p->right;

}

}

return ret;

}