C++使用递归和非递归遍历二叉树

遍历二叉树有三种方式，分别是前序遍历、中序遍历和后序遍历。其中，前序遍历指先遍历根节点，再遍历左子树和右子树；中序遍历指先遍历左子树，再遍历根节点和右子树；后序遍历指先遍历左子树和右子树，再遍历根节点。下面是使用递归和非递归方式遍历二叉树的示例代码：

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

// 二叉树结点的定义
struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

// 递归前序遍历
void preorderTraversalRecursive(TreeNode* root) {
    if (root == NULL) return;
    cout << root->val << " ";
    preorderTraversalRecursive(root->left);
    preorderTraversalRecursive(root->right);
}

// 非递归前序遍历
void preorderTraversalIterative(TreeNode* root) {
    if (root == NULL) return;
    stack<TreeNode*> s;
    s.push(root);
    while (!s.empty()) {
        TreeNode* node = s.top();
        s.pop();
        cout << node->val << " ";
        if (node->right != NULL) s.push(node->right);
        if (node->left != NULL) s.push(node->left);
    }
}

// 递归中序遍历
void inorderTraversalRecursive(TreeNode* root) {
    if (root == NULL) return;
    inorderTraversalRecursive(root->left);
    cout << root->val << " ";
    inorderTraversalRecursive(root->right);
}

// 非递归中序遍历
void inorderTraversalIterative(TreeNode* root) {
    if (root == NULL) return;
    stack<TreeNode*> s;
    TreeNode* node = root;
    while (node != NULL || !s.empty()) {
        while (node != NULL) {
            s.push(node);
            node = node->left;
        }
        node = s.top();
        s.pop();
        cout << node->val << " ";
        node = node->right;
    }
}

// 递归后序遍历
void postorderTraversalRecursive(TreeNode* root) {
    if (root == NULL) return;
    postorderTraversalRecursive(root->left);
    postorderTraversalRecursive(root->right);
    cout << root->val << " ";
}

// 非递归后序遍历
void postorderTraversalIterative(TreeNode* root) {
    if (root == NULL) return;
    stack<TreeNode*> s1, s2;
    s1.push(root);
    while (!s1.empty()) {
        TreeNode* node = s1.top();
        s1.pop();
        s2.push(node);
        if (node->left != NULL) s1.push(node->left);
        if (node->right != NULL) s1.push(node->right);
    }
    while (!s2.empty()) {
        cout << s2.top()->val << " ";
        s2.pop();
    }
}

int main() {
    // 构造二叉树
    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->left = new TreeNode(6);
    root->right->right = new TreeNode(7);

    // 递归遍历
    cout << "递归前序遍历：";
    preorderTraversalRecursive(root);
    cout << endl;
    cout << "递归中序遍历：";
    inorderTraversalRecursive(root);
    cout << endl;
    cout << "递归后序遍历：";
    postorderTraversalRecursive(root);
    cout << endl;

    // 非递归遍历
    cout << "非递归前序遍历：";
    preorderTraversalIterative(root);
    cout << endl;
    cout << "非递归中序遍历：";
    inorderTraversalIterative(root);
    cout << endl;
    cout << "非递归后序遍历：";
    postorderTraversalIterative(root);
    cout << endl;

    return 0;
}

注意，非递归方式需要使用栈来存储待遍历的结点，同时也需要注意遍历顺序。