Recurisve:

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> postorder(Node* root) {
        vector<int> order = {};
        if (root) {
            traversal(root, order);
        }
        
        return order;
    }
    
    void traversal(Node* root, vector<int> &order) {
        int num = root->children.size();
        for (int i = 0; i < num; i++) {
            traversal(root->children.at(i), order);
        }
        
        order.push_back(root->val);
    }
};

　　

Iteratve:

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> postorder(Node* root) {
        vector<int> order = {};
        map<Node*, bool> visit;
        stack<Node*> nodeStack;
        if (root) {
            nodeStack.push(root);
        }
        
        while (!nodeStack.empty()) {
            Node* node = nodeStack.top();
            int num = node->children.size();
            if (num > 0 && visit.find(node) == visit.end()) {
                for (int i = num - 1; i >= 0 ; i--) {
                    nodeStack.push(node->children.at(i));
                }
                visit[node] = true;
            } else {
                order.push_back(node->val);
                nodeStack.pop();
            }
        }
        
        return order;
    }
};