二叉树的建立、各种遍历操作以及树形输出

二叉树的建立可以通过递归或非递归的方式实现，其中递归方式比较简单，可以通过以下代码实现：

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
    if (inorder.empty() || postorder.empty()) {
        return NULL;
    }
    int rootVal = postorder.back();
    TreeNode* root = new TreeNode(rootVal);
    int rootIndex = find(inorder.begin(), inorder.end(), rootVal) - inorder.begin();
    vector<int> leftInorder(inorder.begin(), inorder.begin() + rootIndex);
    vector<int> rightInorder(inorder.begin() + rootIndex + 1, inorder.end());
    vector<int> leftPostorder(postorder.begin(), postorder.begin() + rootIndex);
    vector<int> rightPostorder(postorder.begin() + rootIndex, postorder.end() - 1);
    root->left = buildTree(leftInorder, leftPostorder);
    root->right = buildTree(rightInorder, rightPostorder);
    return root;
}

各种遍历操作包括前序遍历、中序遍历、后序遍历和层序遍历，可以通过递归或非递归的方式实现，以下是递归方式的代码：

void preorderTraversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    cout << root->val << " ";
    preorderTraversal(root->left);
    preorderTraversal(root->right);
}

void inorderTraversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    inorderTraversal(root->left);
    cout << root->val << " ";
    inorderTraversal(root->right);
}

void postorderTraversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    postorderTraversal(root->left);
    postorderTraversal(root->right);
    cout << root->val << " ";
}

void levelOrderTraversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()) {
        int size = q.size();
        for (int i = 0; i < size; i++) {
            TreeNode* node = q.front();
            q.pop();
            cout << node->val << " ";
            if (node->left != NULL) {
                q.push(node->left);
            }
            if (node->right != NULL) {
                q.push(node->right);
            }
        }
        cout << endl;
    }
}

树形输出可以通过递归方式实现，以下是代码：

void printTree(TreeNode* root, int depth) {
    if (root == NULL) {
        return;
    }
    printTree(root->right, depth + 1);
    for (int i = 0; i < depth; i++) {
        cout << "    ";
    }
    cout << root->val << endl;
    printTree(root->left, depth + 1);
}