【二叉树5】前中后遍历二叉树的递归和非递归方法

vector<int> inorderTraversal(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<int> output;
stack<TreeNode *> st;
while (!st.empty() || root != NULL) {
while (root != NULL) {
st.push(root);
root = root->left;
}

if (!st.empty()) {
root = st.top();
output.push_back(root->val);
st.pop();
root = root->right;
}
}
return output;
}