使用数组实现 N叉树 和 N叉树的前序遍历

class Node
{
public:
    int val;
    vector children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector _children) {
        val = _val;
        children = _children;
    }
};

#include 
#include

using std::cout;
using std::endl;
using std::string;
using std::vector;

class Node
{
public:
    int val;
    vector children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector _children) {
        val = _val;
        children = _children;
    }
};

class Solution {
public:
    vector preorder(Node* root) {
        if (root == NULL) return {};
        cout << "enter：" << root->val << endl;
        childarr.push_back(root->val);
        for (Node* child : root->children)
        {
            preorder(child);
        }
        cout << "Leave:" << root->val << endl;
        return childarr;
    }

private:
    vector childarr;
};
//      1
//    / | 
//   3  2  4
//  / 
// 5   6

int main()
{
    //给出N叉树的层序遍历 数组
    vector root = { 1, NULL, 3, 2, 4, NULL, 5, 6 };
    //根据层序遍历的结果 ，构造出多叉树
    Node Tree[6];
    for (int i=0,j=0;i<6 && j<8 ;i++,j++)
    {
        while (root[j]==NULL)
        {
            j++;
        }
        Tree[i].val = root[j];
    }
    Tree[0].children.push_back(&Tree[1]);
    Tree[0].children.push_back(&Tree[2]);
    Tree[0].children.push_back(&Tree[3]);
    Tree[1].children.push_back(&Tree[4]);
    Tree[1].children.push_back(&Tree[5]);

    Solution todo;
    //N叉树的前序遍历
    vector result = todo.preorder(Tree);

    //前序遍历的结果 打印
    for (auto it = result.begin();it!=result.end();it++)
    {
        cout<<" "<<*it;
    }
}

 结果中，打印出的enter  和 Leave 是 深度优先遍历的 进入 和 回溯 的节点。 

在 for 循环里面和外面 唯一的区别就 是对根节点的处理。

如果更改成在for循环里面打印，结果如下：

class Solution {
public:
    vector preorder(Node* root) {
        if (root == NULL) return {};
        
        childarr.push_back(root->val);
        //在 for 循环里 打印
        for (Node* child : root->children)
        {
            cout << "enter：" << child->val << endl;
            preorder(child);
            cout << "Leave:" << child->val << endl;
        }
       
        return childarr;
    }

private:
    vector childarr;
};