590. N-ary Tree Postorder Traversal

dfs O(n) time O(h) space

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

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<int> postorder(Node* root) {
        dfs(root);
        return res;
    }

    void dfs(Node *u) {
        if (!u) return;
        for (auto v : u->children) {
            dfs(v);
        }
        res.push_back(u->val);
    }

    vector<int> res;
};

iterative

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

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<int> postorder(Node* root) {
        vector<int> res;
        stack<pair<Node *, bool>> s;
        s.emplace(root, false);
        while (!empty(s)) {
            auto [u, visited] = s.top(); s.pop();
            if (!u) continue;
            if (visited) {
                res.push_back(u->val);
                continue;
            }
            s.emplace(u, true);
            for (auto it = rbegin(u->children); it != rend(u->children); ++it) {
                s.emplace(*it, false);
            }
        }
        return res;
    }
};