> For the complete documentation index, see [llms.txt](https://docs.1l.lol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.1l.lol/search/tree-traversal.md).

# 二叉树的遍历与搜索

### 四种遍历

* **先序**（根左右）：用于复制树
* **中序**（左根右）：BST 中序得到有序序列
* **后序**（左右根）：用于释放内存
* **层序**（逐层）：借助队列

```cpp
struct Node { int val; Node *left, *right; };

void preorder(Node* r) { if (!r) return; cout << r->val << " "; preorder(r->left); preorder(r->right); }
void inorder(Node* r)  { if (!r) return; inorder(r->left); cout << r->val << " "; inorder(r->right); }
void postorder(Node* r){ if (!r) return; postorder(r->left); postorder(r->right); cout << r->val << " "; }

void levelOrder(Node* root) {
    if (!root) return;
    queue<Node*> q; q.push(root);
    while (!q.empty()) {
        Node* cur = q.front(); q.pop();
        cout << cur->val << " ";
        if (cur->left)  q.push(cur->left);
        if (cur->right) q.push(cur->right);
    }
}
```

### 由遍历序列还原二叉树

* 已知**先序 + 中序**，可唯一确定一棵二叉树；
* 已知**后序 + 中序**，也可唯一确定；
* 仅知先序 + 后序一般**不能**唯一确定。

### 二叉树的搜索

在 BST 中查找某值，根据大小关系决定往左还是往右，平均 O(log n)，即“二叉树的搜索算法”。


---

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