1.树层次遍历算法，返回树深度

#include <iostream>
#include <queue>struct TreeNode {int val;TreeNode* left;TreeNode* right;TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};int maxDepth(TreeNode* root) {if (root == nullptr) {return 0;}std::queue<TreeNode*> q;q.push(root);int depth = 0;while (!q.empty()) {int levelSize = q.size();depth++; // 进入下一层for (int i = 0; i < levelSize; i++) {TreeNode* node = q.front();q.pop();if (node->left != nullptr) {q.push(node->left);}if (node->right != nullptr) {q.push(node->right);}}}return depth;
}int main() {// 构建一个二叉树TreeNode* root = new TreeNode(1);root->left = new TreeNode(2);root->right = new TreeNode(3);root->left->left = new TreeNode(4);root->left->right = new TreeNode(5);root->right->right = new TreeNode(6);std::cout << "Depth of the tree: " << maxDepth(root) << std::endl;// 释放内存，防止内存泄漏delete root->left->left;delete root->left->right;delete root->right->right;delete root->left;delete root->right;delete root;return 0;
}

2.树前序遍历（非递归）

#include <iostream>
#include <stack>
#include <vector>struct TreeNode {int val;TreeNode* left;TreeNode* right;TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};std::vector<int> preorderTraversal(TreeNode* root) {std::vector<int> result;std::stack<TreeNode*> stk;TreeNode* current = root;while (current != nullptr || !stk.empty()) {if (current != nullptr) {// 访问当前节点result.push_back(current->val);// 将当前节点的右子树压栈if (current->right != nullptr) {stk.push(current->right);}// 继续访问当前节点的左子树current = current->left;} else {// 当前节点为空时，弹出栈顶节点，并将其赋值给 currentcurrent = stk.top();stk.pop();}}return result;
}int main() {// 构建一个二叉树TreeNode* root = new TreeNode(1);root->left = new TreeNode(2);root->right = new TreeNode(3);root->left->left = new TreeNode(4);root->left->right = new TreeNode(5);// 非递归前序遍历二叉树std::vector<int> result = preorderTraversal(root);// 输出遍历结果std::cout << "Preorder traversal result: ";for (int num : result) {std::cout << num << " ";}std::cout << std::endl;// 释放内存delete root->left->left;delete root->left->right;delete root->left;delete root->right;delete root;return 0;
}

3.树中序遍历（非递归）

#include <iostream>
#include <stack>
#include <vector>struct TreeNode {int val;TreeNode* left;TreeNode* right;TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};std::vector<int> inorderTraversal(TreeNode* root) {std::vector<int> result;std::stack<TreeNode*> stk;TreeNode* current = root;while (current != nullptr || !stk.empty()) {if (current != nullptr) {// 将当前节点及其左子树压栈stk.push(current);current = current->left;} else {// 当前节点为空时，弹出栈顶节点并访问，然后继续遍历其右子树current = stk.top();stk.pop();result.push_back(current->val);current = current->right;}}return result;
}int main() {// 构建一个二叉树TreeNode* root = new TreeNode(1);root->left = new TreeNode(2);root->right = new TreeNode(3);root->left->left = new TreeNode(4);root->left->right = new TreeNode(5);// 非递归中序遍历二叉树std::vector<int> result = inorderTraversal(root);// 输出遍历结果std::cout << "Inorder traversal result: ";for (int num : result) {std::cout << num << " ";}std::cout << std::endl;// 释放内存delete root->left->left;delete root->left->right;delete root->left;delete root->right;delete root;return 0;
}

4.树后序遍历（非递归）

#include <iostream>
#include <stack>
#include <vector>struct TreeNode {int val;TreeNode* left;TreeNode* right;TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};std::vector<int> postorderTraversal(TreeNode* root) {std::vector<int> result;std::stack<TreeNode*> stk;TreeNode* current = root;TreeNode* lastVisited = nullptr;while (current != nullptr || !stk.empty()) {if (current != nullptr) {// 将当前节点及其左子树压栈stk.push(current);current = current->left;} else {TreeNode* topNode = stk.top();// 如果当前节点的右子树为空，或者已经访问过了右子树，那么就可以访问当前节点if (topNode->right == nullptr || topNode->right == lastVisited) {result.push_back(topNode->val);lastVisited = topNode;stk.pop();} else {// 否则，继续遍历右子树current = topNode->right;}}}return result;
}int main() {// 构建一个二叉树TreeNode* root = new TreeNode(1);root->left = new TreeNode(2);root->right = new TreeNode(3);root->left->left = new TreeNode(4);root->left->right = new TreeNode(5);// 非递归后序遍历二叉树std::vector<int> result = postorderTraversal(root);// 输出遍历结果std::cout << "Postorder traversal result: ";for (int num : result) {std::cout << num << " ";}std::cout << std::endl;// 释放内存delete root->left->left;delete root->left->right;delete root->left;delete root->right;delete root;return 0;
}