diff --git a/include/offer_05.hpp b/include/offer_05.hpp
new file mode 100644
index 0000000..3b4976c
--- /dev/null
+++ b/include/offer_05.hpp
@@ -0,0 +1,13 @@
+#ifndef OFFER_05_HPP
+#define OFFER_05_HPP
+
+#include <string>
+
+using namespace std;
+
+class Solution {
+ public:
+  string replaceSpace(string s);
+};
+
+#endif
diff --git a/notes/src/SUMMARY.md b/notes/src/SUMMARY.md
index af4619f..3213214 100644
--- a/notes/src/SUMMARY.md
+++ b/notes/src/SUMMARY.md
@@ -4,6 +4,10 @@
 - [移除元素](./remove_elements.md)
 - [长度最小的子数组](./minimum_size_subarray_sum.md)
 
+# 字符串
+
+- [替换空格](./substitute_spaces.md)
+
 # 经典代码
 
 - [合并两个有序链表](./merge_two_sorted_linked_lists.md)
diff --git a/notes/src/substitute_spaces.md b/notes/src/substitute_spaces.md
new file mode 100644
index 0000000..75956f8
--- /dev/null
+++ b/notes/src/substitute_spaces.md
@@ -0,0 +1,15 @@
+# 替换空格
+
+[Leetcode](https://leetcode.cn/problems/ti-huan-kong-ge-lcof/)
+
+请实现一个函数，把字符串 s 中的每个空格替换成"%20"。
+
+示例 1：
+
+> 输入：s = "We are happy."
+>
+> 输出："We%20are%20happy."
+
+![demo](https://tva1.sinaimg.cn/large/e6c9d24ely1go6qmevhgpg20du09m4qp.gif)
+
+对于很多数组填充类问题，都可以先计算出扩展后数组的长度，然后从后往前双指针。
diff --git a/src/offer_05.cpp b/src/offer_05.cpp
new file mode 100644
index 0000000..86d9b1a
--- /dev/null
+++ b/src/offer_05.cpp
@@ -0,0 +1,24 @@
+#include "offer_05.hpp"
+
+string Solution::replaceSpace(string s) {
+  int len = s.length();
+  int cnt{0};
+  for (int i{0}; i < len; ++i) {
+    if (s[i] == ' ') {
+      s.push_back(' ');
+      s.push_back(' ');
+      cnt++;
+    }
+  }
+  len = s.length();
+  for (int fast = len - 1, slow = len - 1 - 2 * cnt; fast >= 0; --slow) {
+    if (s[slow] == ' ') {
+      s[fast--] = '0';
+      s[fast--] = '2';
+      s[fast--] = '%';
+    } else {
+      s[fast--] = s[slow];
+    }
+  }
+  return s;
+}
diff --git a/tests/offer_05.cpp b/tests/offer_05.cpp
new file mode 100644
index 0000000..cb83dc0
--- /dev/null
+++ b/tests/offer_05.cpp
@@ -0,0 +1,10 @@
+#include "offer_05.hpp"
+
+#include <gtest/gtest.h>
+
+TEST(Offer05, Case1) {
+  string s{"We are happy."};
+  string expected{"We%20are%20happy."};
+  Solution solution;
+  EXPECT_EQ(solution.replaceSpace(s), expected);
+}