letcode 分类练习 树的遍历

树的构建

输入数组：[8, 3, 10, 1, 6, null, 14, null, null, 4, 7, 13]

#include <iostream>
#include <vector>
#include <queue>
#include <memory>

using namespace std;

// 定义二叉树节点结构
struct TreeNode {
   
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {
   }
};

// 根据向量数组构建二叉树
TreeNode* constructBinaryTree(const vector<int*>& nums) {
   
    if (nums.empty() || !nums[0]) {
   
        return nullptr;
    }

    // 使用队列进行层序遍历构建树
    queue<TreeNode*> q;
    TreeNode* root = new TreeNode(*nums[0]);
    q.push(root);
    int i = 1;

    while (!q.empty() && i < nums.size()) {
   
        TreeNode* current = q.front();
        q.pop();

        // 构建左子节点
        if (i < nums.size() && nums[i]) {
   
            current->left = new TreeNode(*nums[i]);
            q.push(current->left);
        }
        i++;

        // 构建右子节点
        if (i < nums.size() && nums[i]) {
   
            current->right = new TreeNode(*nums[i]);
            q.push(current->right);
        }
        i++;
    }

    return root;
}

递归遍历

前序遍历

class Solution {
   
public:
    vector<int> result;
    void dfs(TreeNode* root){
   
        if(!root) return;
        result.push_back(root->val);
        if(root -> left)dfs(root -> left);
        if(root -> right)dfs(root-> right);
    }
    vector<int> preorderTraversal(TreeNode* root) {
   
        dfs(root);
        return result;
    }
};

中序遍历

class Solution {
   
public:
    vector<int> result;
    void dfs(TreeNode* root){
   
        if(!root) return;
        if(root->left)dfs(root->left);
        result.push_back(root -> val);
        if(root->right)dfs(root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
   
        dfs(root);
        return result;
    }
};

后序遍历

class Solution {
   
public:
    vector<int> result;
    void dfs(TreeNode* root){
   
        if(!root) return;
        if(root->left)dfs(root->left);
        if(root->right)dfs(root->right);
        result.push_back(root -> val);