Leetcode - 589 - N-ary Tree Preorder Traversal

Recursive:

Java:

class Solution {
    public List<Integer> list = new ArrayList<>();
    public List<Integer> preorder(Node root) {
        if (root == null)
            return list;
        
        list.add(root.val);
        for(Node node: root.children)
            preorder(node);
                
        return list;
    }
}

// https://leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/147955/Java-Iterative-and-Recursive-Solutions
           

C++:

class Solution {
private:
    void travel(Node* root, vector<int>& result) {
        if (root == nullptr) {
            return;
        }
        
        result.push_back(root -> val);
        for (Node* child : root -> children) {
            travel(child, result);
        }
    }
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        travel(root, result);
        return result;
    }
};

// https://leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/149093/C%2B%2B-44ms-beats-100-both-iterative-and-recursive
           

Iterative:

add the same level from right to the left and save the children of the left most node

Java:

class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        
        Stack<Node> stack = new Stack<>();
        stack.add(root);
        
        while (!stack.empty()) {
            root = stack.pop();
            list.add(root.val);
            for (int i = root.children.size() - 1; i >= 0; i--)
                stack.add(root.children.get(i));
        }
        
        return list;
    }
}

// https://leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/147955/Java-Iterative-and-Recursive-Solutions
           

C++:

class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        if (root == nullptr) {
            return result;
        }
        
        stack<Node*> stk;
        stk.push(root);
        while (!stk.empty()) {
            Node* cur = stk.top();
            stk.pop();
            result.push_back(cur -> val);
            for (int i = cur -> children.size() - 1; i >= 0; i--) {
                if (cur -> children[i] != nullptr) {
                    stk.push(cur -> children[i]);
                }
            }
        }
        return result;
    }
};

// https://leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/149093/C%2B%2B-44ms-beats-100-both-iterative-and-recursive
           

Python:

q += [child for child in node.children[::-1] if child]
           

class Solution(object):
    def preorder(self, root):
        ret, q = [], root and [root]
        while q:
            node = q.pop()
            ret.append(node.val)
            q += [child for child in node.children[::-1] if child]
        return ret

# https://leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/148867/Python-short-iterative-solution-beats-100-66-ms-faster-than-fastest-!