feat(common,src): add preorder traversal print function and 99-recoverTree

Author: kwu130

include/common.h

@@ -105,6 +105,31 @@ inline TreeNode* build(const std::vector<int>& vec, int nullptr_val = -1) {
     return root;
 }
 
+// preorder
+inline void print(const TreeNode* root) {
+    if (root == nullptr) {
+        std::cout << "null" << std::endl;
+        return;
+    }
+
+    std::queue<const TreeNode*> q;
+    q.push(root);
+
+    while (!q.empty()) {
+        const TreeNode* cur = q.front();
+        q.pop();
+
+        if (cur == nullptr) {
+            std::cout << "null ";
+        } else {
+            std::cout << cur->val << " ";
+            q.push(cur->left);
+            q.push(cur->right);
+        }
+    }
+    std::cout << std::endl;
+}
+
 inline bool equal(const TreeNode* a, const TreeNode* b) {
     if (a == nullptr && b == nullptr) return true;
     if (a == nullptr || b == nullptr) return false;

src/99-recoverTree.cpp

@@ -0,0 +1,63 @@
+//
+// Created by 1412 on 2025/11/29.
+//
+
+#include <bits/stdc++.h>
+#include "common.h"
+
+using namespace std;
+using TreeNode = treeHelper::TreeNode;
+
+class Solution {
+public:
+    void recoverTree(TreeNode* root) {
+        TreeNode *first = nullptr, *second = nullptr;
+        inorder(root, first, second);
+        swap(first->val, second->val);
+    }
+
+private:
+    void inorder(TreeNode* root, TreeNode*& first, TreeNode*& second) {
+        TreeNode* prev = nullptr;
+        stack<TreeNode*> s;
+        while (root != nullptr || !s.empty()) {
+            while (root != nullptr) {
+                s.push(root);
+                root = root->left;
+            }
+            root = s.top();
+            s.pop();
+            if (prev != nullptr && prev->val > root->val) {
+                second = root;
+                if (first == nullptr) {
+                    first = prev;
+                }
+            }
+            prev = root;
+            root = root->right;
+        }
+    }
+};
+
+void test() {
+    Solution s;
+    int nullptr_value = -1;
+    TreeNode* root = treeHelper::build({1, 3, nullptr_value, nullptr_value, 2}, nullptr_value);
+    TreeNode* expected = treeHelper::build({3, 1, nullptr_value, nullptr_value, 2}, nullptr_value);
+    s.recoverTree(root);
+    assert(treeHelper::equal(root, expected));
+    treeHelper::free(root);
+    treeHelper::free(expected);
+
+    root = treeHelper::build({3, 1, 4, nullptr_value, nullptr_value, 2}, nullptr_value);
+    expected = treeHelper::build({2, 1, 4, nullptr_value, nullptr_value, 3}, nullptr_value);
+    s.recoverTree(root);
+    assert(treeHelper::equal(root, expected));
+    treeHelper::free(root);
+    treeHelper::free(expected);
+}
+
+int main() {
+    test();
+    return 0;
+}