vector<int> postorderTraversal(TreeNode *root) {

vector<int> res;

if(!root) {

return res;

}

stack<TreeNode* > s;

s.push(root);

TreeNode* child = root;

while(!s.empty()) {

TreeNode* top = s.top();

if( (!top->left&&!top->right) || top->left == child || top->right == child) {

res.push_back(top->val);

child = top;

s.pop();

continue;

}

if(top->right)

s.push(top->right);

if(top->left)

s.push(top->left);

child=top;

}

return res;

}