解释这行代码if(k<a. length()-1) preorder(a. substr(k+1,a. length()-k-1),b. substr( k,b. length()-k-1));

这行代码是一个递归函数的调用，它的作用是对字符串a和b进行前序遍历。其中，k是一个索引值，表示当前遍历到的位置。如果k小于a的长度减1，那么就对a和b的子串进行前序遍历。具体实现可以参考代码中的preorder函数。

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解释这段代码void preorder(string a,string b){ if(b. length() ==1) pre += b; else { int k = a. find(b. substr(b. length()-1,1)); pre += a[k]; if(k>0) preorder(a. substr(0,k),b. substr(0,k)); if(k<a. length()-1) preorder(a. substr(k+1,a. length()-k-1),b. substr( k,b. length()-k-1)); } }

这段代码是一个二叉树的前序遍历函数，其中a和b分别表示二叉树的前序遍历序列和中序遍历序列。如果中序遍历序列的长度为1，则将其作为当前节点的值。否则，找到中序遍历序列中最后一个元素在前序遍历序列中的位置k，将a[k]作为当前节点的值。然后递归地对左子树和右子树进行前序遍历，直到遍历完整个二叉树。

代码改进，不使用cout<<endl结构#include <iostream> #include <queue> #include <stack> #include <string> using namespace std; struct Student { string name; int number; int score; }; struct TreeNode { Student data; TreeNode* left; TreeNode* right; TreeNode(Student s) : data(s), left(nullptr), right(nullptr) {} }; void postOrder(TreeNode* root) { if (!root) return; postOrder(root->left); postOrder(root->right); cout << root->data.name << " " << root->data.number << " " << root->data.score << endl; } void preOrder(TreeNode* root) { stack<TreeNode*> s; s.push(root); while (!s.empty()) { TreeNode* node = s.top(); s.pop(); if (node) { cout << node->data.name << " " << node->data.number << " " << node->data.score << endl; s.push(node->right); s.push(node->left); } } } void levelOrder(TreeNode* root) { queue<TreeNode*> q; q.push(root); while (!q.empty()) { TreeNode* node = q.front(); q.pop(); if (node) { cout << node->data.name << " " << node->data.number << " " << node->data.score << endl; q.push(node->left); q.push(node->right); } } } int main() { Student students[] = { {"Lei Zhenzi", 101401, 82}, {"Jiang Ziya", 100032, 90}, {"Ne Zha", 101674, 70}, {"Shen Gongbao", 101982, 87}, {"Jiu Weihu", 107431, 75}, {"Tian Zun", 100001, 98}, {"Tai Yi", 101009, 81}, {"Yang Jian", 101321, 63}, {"Huang Feihu", 101567, 72}, {"Zhou Wang", 108160, 55}, {"Li Jing", 102456, 84}, {"Tu Xingsun", 102245, 65}, }; int n = sizeof(students) / sizeof(Student); TreeNode* root = new TreeNode(students[0]); queue<TreeNode*> q; q.push(root); for (int i = 1; i < n; i += 2) { TreeNode* parent = q.front(); q.pop(); parent->left = new TreeNode(students[i]); q.push(parent->left); if (i + 1 < n) { parent->right = new TreeNode(students[i + 1]); q.push(parent->right); } } cout << "Post-order traversal: " << endl; postOrder(root); cout << "Pre-order traversal: " << endl; preOrder(root); cout << "Level-order traversal: " << endl; levelOrder(root); return 0; }

可以使用'\n'代替endl，这样可以避免刷新缓存区的操作，提高输出效率。修改后的代码如下：

#include <iostream>
#include <queue>
#include <stack>
#include <string>

using namespace std;

struct Student {
    string name;
    int number;
    int score;
};

struct TreeNode {
    Student data;
    TreeNode* left;
    TreeNode* right;
    TreeNode(Student s) : data(s), left(nullptr), right(nullptr) {}
};

void postOrder(TreeNode* root) {
    if (!root) return;
    postOrder(root->left);
    postOrder(root->right);
    cout << root->data.name << " " << root->data.number << " " << root->data.score << '\n';
}

void preOrder(TreeNode* root) {
    stack<TreeNode*> s;
    s.push(root);
    while (!s.empty()) {
        TreeNode* node = s.top();
        s.pop();
        if (node) {
            cout << node->data.name << " " << node->data.number << " " << node->data.score << '\n';
            s.push(node->right);
            s.push(node->left);
        }
    }
}

void levelOrder(TreeNode* root) {
    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()) {
        TreeNode* node = q.front();
        q.pop();
        if (node) {
            cout << node->data.name << " " << node->data.number << " " << node->data.score << '\n';
            q.push(node->left);
            q.push(node->right);
        }
    }
}

int main() {
    Student students[] = {
        {"Lei Zhenzi", 101401, 82},
        {"Jiang Ziya", 100032, 90},
        {"Ne Zha", 101674, 70},
        {"Shen Gongbao", 101982, 87},
        {"Jiu Weihu", 107431, 75},
        {"Tian Zun", 100001, 98},
        {"Tai Yi", 101009, 81},
        {"Yang Jian", 101321, 63},
        {"Huang Feihu", 101567, 72},
        {"Zhou Wang", 108160, 55},
        {"Li Jing", 102456, 84},
        {"Tu Xingsun", 102245, 65},
    };
    int n = sizeof(students) / sizeof(Student);

    TreeNode* root = new TreeNode(students[0]);
    queue<TreeNode*> q;
    q.push(root);
    for (int i = 1; i < n; i += 2) {
        TreeNode* parent = q.front();
        q.pop();
        parent->left = new TreeNode(students[i]);
        q.push(parent->left);
        if (i + 1 < n) {
            parent->right = new TreeNode(students[i + 1]);
            q.push(parent->right);
        }
    }

    cout << "Post-order traversal: \n";
    postOrder(root);

    cout << "Pre-order traversal: \n";
    preOrder(root);

    cout << "Level-order traversal: \n";
    levelOrder(root);

    return 0;
}