590. N-ary Tree Postorder Traversal

590. N-ary Tree Postorder Traversal

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题目

Leetcode题目

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

For example, given a 3-ary tree:

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

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解决

设一共有n个节点。

后序遍历的递归过程：

1. 从左至右依次遍历所有子树
2. 访问根节点

1.递归

• 时间复杂度：O(n)
• 空间复杂度：O(n)

/*
// 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> result;
        traversal(root, result);
        return result;
    }

    void traversal(Node* root, vector<int> & num) {
        if (!root) return;
        for (auto n : root->children) {
            traversal(n, num);
        }
        num.push_back(root->val);
    }
};

2.非递归

• 时间复杂度：O(n)
• 空间复杂度：O(n)

/*
// 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> result;
        stack<Node*> tree;
        if (!root) return result;
        tree.push(root);
        while (!tree.empty()) {
            Node* temp = tree.top();
            tree.pop();
            for (auto node : temp->children) {
                tree.push(node);
            }
            result.push_back(temp->val);
        }
        reverse(result.begin(), result.end());
        return result;
    }
};