diff --git a/include/RedBlackTree.h b/include/RedBlackTree.h
index 714a13b..12d8e80 100644
--- a/include/RedBlackTree.h
+++ b/include/RedBlackTree.h
@@ -130,20 +130,20 @@ namespace ling {
 
         /// 查找元素
         /// 区别于findSearch，此方法会在没有匹配时返回接近的节点
-        Node *iterativeSearch(T key);
+        Node *iterativeSearch(T key) const;
 
-        Node *findSearch(T key);
+        Node *findSearch(T key) const;
 
         /// 根节点
-        Node *getRoot() {
+        Node *getRoot() const {
             return rootNode;
         }
 
         // 查找最小结点：返回tree为根结点的红黑树的最小结点。
-        Node *minimum();
+        Node *minimum() const;
 
         // 查找最大结点：返回tree为根结点的红黑树的最大结点。
-        Node *maximum();
+        Node *maximum() const;
 
 #define rb_parent(r)   ((r)->parent)
 #define rb_color(r) ((r)->color)
@@ -157,7 +157,7 @@ namespace ling {
     };
 
     template<typename T>
-    typename RedBlackTree<T>::Node *RedBlackTree<T>::maximum() {
+    typename RedBlackTree<T>::Node *RedBlackTree<T>::maximum() const {
         Node *tree = rootNode;
         if (tree == nullptr)
             return nullptr;
@@ -168,7 +168,7 @@ namespace ling {
     }
 
     template<typename T>
-    typename RedBlackTree<T>::Node *RedBlackTree<T>::minimum() {
+    typename RedBlackTree<T>::Node *RedBlackTree<T>::minimum() const {
         Node *tree = rootNode;
         if (tree == nullptr)
             return nullptr;
@@ -366,12 +366,12 @@ namespace ling {
     }
 
     template<typename T>
-    typename RedBlackTree<T>::Node *RedBlackTree<T>::findSearch(T key) {
+    typename RedBlackTree<T>::Node *RedBlackTree<T>::findSearch(T key) const {
         return findSearch(rootNode, key);
     }
 
     template<typename T>
-    typename RedBlackTree<T>::Node *RedBlackTree<T>::iterativeSearch(T key) {
+    typename RedBlackTree<T>::Node *RedBlackTree<T>::iterativeSearch(T key) const {
         return iterativeSearch(rootNode, key);
     }