589. N-ary Tree Preorder Traversal

 

Given an n-ary tree, return the  preorder  traversal of its nodes’ values.

For example, given a 3-ary tree:

 

 

Return its preorder traversal as: [1,3,5,6,2,4].

 

Note:

Recursive solution is trivial, could you do it iteratively?

 

这道题让我们求N叉树的前序遍历,有之前那道Binary Tree Preorder Traversal的基础,知道了二叉树的前序遍历的方法,很容易就可以写出N叉树的前序遍历。先来看递归的解法,主要实现一个递归函数即可,判空之后,将当前结点值加入结果res中,然后遍历子结点数组中所有的结点,对每个结点都调用递归函数即可,参见代码如下:

 

解法一:

class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> res;
        helper(root, res);
        return res;
    }
    void helper(Node* node, vector<int>& res) {
        if (!node) return;
        res.push_back(node->val);
        for (Node* child : node->children) {
            helper(child, res);
        }
    }
};

 

我们也可以使用迭代的解法来做,使用栈stack来辅助,需要注意的是,如果使用栈的话,我们遍历子结点数组的顺序应该是从后往前的,因为栈是后进先出的顺序,所以需要最先遍历的子结点应该最后进栈,参见代码如下:

 

解法二:

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

 

类似题目:

Binary Tree Preorder Traversal

N-ary Tree Level Order Traversal

N-ary Tree Postorder Traversal

 

参考资料:

https://leetcode.com/problems/n-ary-tree-preorder-traversal/

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

 

LeetCode All in One 题目讲解汇总(持续更新中…) 


转载请注明来源于 Grandyang 的博客 (grandyang.com),欢迎对文章中的引用来源进行考证,欢迎指出任何有错误或不够清晰的表达。可以在下面评论区评论,也可以邮件至 grandyang@qq.com

💰


微信打赏


Venmo 打赏

×

Help us with donation