树递归无限进行

Question

我编写了一个从向量构造二叉树的代码。问题是当我调用inOrder(root) 时，它会进入无限递归，即root 永远不会变成nullptr。

#include <iostream>
#include <vector>
using namespace std;

class Node {
public:
    int data;
    Node* left;
    Node* right;
    explicit Node(int element) {
        data = element;
        left = nullptr;
        right= nullptr;
    }
};

Node* construct_tree(Node* root, vector<int> &vec, int i) {
    if(i < vec.size()) {
        Node* new_node = new Node(vec[i]);
        root = new_node;
        root->left = construct_tree(root, vec, 2*i + 1);
        root->right = construct_tree(root, vec, 2*i + 2);
    }
    return root;
}

void inOrder(Node* root) {
    if (root != nullptr) {
        inOrder(root->left);
        int n = root->data;
        cout << n << " ";
        inOrder(root->right);
    }
}

int main() {
    vector<int> vec{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    Node *root = nullptr;
    root = construct_tree(root, vec, 0);
    cout << root->data << " ";
    cout << root->left->data;
    inOrder(root);
    return 0;
}

可能的修复？

Answer 1

你是从construct_tree返回root，所以当i < vec.size()为假时，它可能会将root分配给root->left或root->right，创建循环。

在这种情况下，您将不需要参数 root，因为它的值从未在函数 construct_tree 中使用。

试试这个：

#include <iostream>
#include <vector>
using namespace std;

class Node {
public:
    int data;
    Node* left;
    Node* right;
    explicit Node(int element) {
        data = element;
        left = nullptr;
        right= nullptr;
    }
};

Node* construct_tree( vector<int> &vec, int i) {
    if(i < vec.size()) {
        Node* new_node = new Node(vec[i]);
        // stop using root and use new_node directly
        new_node->left = construct_tree(vec, 2*i + 1);
        new_node->right = construct_tree(vec, 2*i + 2);
        return new_node;
    } else {
        // return nullptr when the condition is false
        return nullptr;
    }
}

void inOrder(Node* root) {
    if (root != nullptr) {
        inOrder(root->left);
        int n = root->data;
        cout << n << " ";
        inOrder(root->right);
    }
}

int main() {
    vector<int> vec{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    Node *root = nullptr;
    root = construct_tree(vec, 0); // remove the argument root
    cout << root->data << " ";
    cout << root->left->data;
    inOrder(root);
    return 0;
}