LeetCode Binary Tree Postorder Traversal

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> path;
        vector<TreeNode*> stack;
        if (root != NULL) {
            stack.push_back(root);
        }
        TreeNode* last = NULL;
        
        while(!stack.empty()) {
            TreeNode* cur = stack.back();
            
            if (cur->left == NULL && cur->right == NULL) {
                path.push_back(cur->val);
                stack.pop_back();
                last = cur;
                continue;
            }
            
            if (last != NULL && (cur->left == last || cur->right == last)) {
                path.push_back(cur->val);
                stack.pop_back();
                last = cur;
                continue;
            }
            
            while (cur != NULL) {
                if (cur->right != NULL) stack.push_back(cur->right);
                if (cur->left != NULL) stack.push_back(cur->left);
                cur = cur->left;
            }
        }
        
        return path;
    }
};