DFS&Iteration Binary Tree Postorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    void dfs(vector<int> &ans, TreeNode* node) {
        if(node == nullptr) return;
        if(node->left != nullptr) {
            dfs(ans, node->left);
        }
        if(node->right != nullptr) {
            dfs(ans, node->right);
        }
        ans.push_back(node->val);
    }
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        dfs(ans, root);
        return ans;
    }
};

/**
 * Definition for a binary tree node.
 * 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> ans;
        TreeNode *p, *q;
        stack<TreeNode*> s;
        p = root;
        do {
            while(p != nullptr) {
                s.push(p);
                p = p->left;
            }
            q = nullptr;
            while(!s.empty()) {
                p = s.top();
                s.pop();
                if(p->right == q) {
                    ans.push_back(p->val);
                    q = p;
                }else {
                    s.push(p);
                    p = p->right;
                    break;
                }
            }
        }while(!s.empty());
        return ans;
    }
};