diff --git a/include/s0151_reverse_words_in_a_string.hpp b/include/s0151_reverse_words_in_a_string.hpp
new file mode 100644
index 0000000..c8e4977
--- /dev/null
+++ b/include/s0151_reverse_words_in_a_string.hpp
@@ -0,0 +1,13 @@
+#ifndef S0151_REVERSE_WORDS_IN_A_STRING_HPP
+#define S0151_REVERSE_WORDS_IN_A_STRING_HPP
+
+#include <string>
+
+using namespace std;
+
+class Solution {
+ public:
+  string reverseWords(string s);
+};
+
+#endif
diff --git a/notes/src/SUMMARY.md b/notes/src/SUMMARY.md
index 3213214..b7bd268 100644
--- a/notes/src/SUMMARY.md
+++ b/notes/src/SUMMARY.md
@@ -7,6 +7,7 @@
 # 字符串
 
 - [替换空格](./substitute_spaces.md)
+- [翻转字符串里的单词](./reverse_words_in_a_string.md)
 
 # 经典代码
 
diff --git a/notes/src/reverse_words_in_a_string.md b/notes/src/reverse_words_in_a_string.md
new file mode 100644
index 0000000..f1ba1a9
--- /dev/null
+++ b/notes/src/reverse_words_in_a_string.md
@@ -0,0 +1,27 @@
+# 翻转字符串里的单词
+
+[Leetcode](https://leetcode.com/problems/reverse-words-in-a-string/)
+
+1. 去除单词中的额外空格
+2. 翻转整个字符串
+3. 挨个翻转单词
+
+双指针去除额外空格：
+
+如果快指针指向的是空格则跳过；
+
+如果快指针指向的不是空格则把 `s[fast]` 覆盖到 `s[slow]`；
+
+如果快指针指向的是空格并且下一个指向的不是空格，则在 `s[slow]` 处添加一个新空格（前提是 `slow` 不为 `0`，因为这种情况代表字符串的开头是空格）。
+
+翻转字符串：
+
+```cpp
+void reverseSubStr(string &s, int begin, int end) {
+  for (; begin < end; ++begin, --end) {
+    auto tmp = s[begin];
+    s[begin] = s[end];
+    s[end] = tmp;
+  }
+}
+```
diff --git a/src/s0151_reverse_words_in_a_string.cpp b/src/s0151_reverse_words_in_a_string.cpp
new file mode 100644
index 0000000..29082e2
--- /dev/null
+++ b/src/s0151_reverse_words_in_a_string.cpp
@@ -0,0 +1,49 @@
+#include "s0151_reverse_words_in_a_string.hpp"
+
+// reverse a substring
+void reverseSubStr(string &s, int begin, int end) {
+  for (; begin < end; ++begin, --end) {
+    auto tmp = s[begin];
+    s[begin] = s[end];
+    s[end] = tmp;
+  }
+}
+
+string Solution::reverseWords(string s) {
+  if (s.length() == 0) {
+    return s;
+  }
+  // remove extra spaces
+  int len = s.length();
+  int slow{0};
+  for (int fast{0}; fast < len; ++fast) {
+    if (s[fast] != ' ') {
+      s[slow++] = s[fast];
+    } else {
+      if (fast == len - 1) {
+        break;
+      } else {
+        if (s[fast + 1] != ' ' && slow != 0) {
+          s[slow++] = ' ';
+        }
+      }
+    }
+  }
+  s.resize(slow);
+  // reverse the whole string
+  len = s.length();
+  reverseSubStr(s, 0, len - 1);
+  // reverse each words
+  for (int begin{0}, end{0}; end < len; ++end) {
+    if (end == len - 1) {
+      reverseSubStr(s, begin, end);
+    } else {
+      if (s[end + 1] == ' ') {
+        reverseSubStr(s, begin, end);
+        begin = end + 2;
+        end = end + 1;
+      }
+    }
+  }
+  return s;
+}
diff --git a/tests/s0151_reverse_words_in_a_string.cpp b/tests/s0151_reverse_words_in_a_string.cpp
new file mode 100644
index 0000000..a69ae8a
--- /dev/null
+++ b/tests/s0151_reverse_words_in_a_string.cpp
@@ -0,0 +1,31 @@
+#include "s0151_reverse_words_in_a_string.hpp"
+
+#include <gtest/gtest.h>
+
+TEST(Problem151, Case1) {
+  string s{"the sky is blue"};
+  string expected{"blue is sky the"};
+  Solution solution;
+  EXPECT_EQ(solution.reverseWords(s), expected);
+}
+
+TEST(Problem151, Case2) {
+  string s{"  hello world  "};
+  string expected{"world hello"};
+  Solution solution;
+  EXPECT_EQ(solution.reverseWords(s), expected);
+}
+
+TEST(Problem151, Case3) {
+  string s{"a good   example"};
+  string expected{"example good a"};
+  Solution solution;
+  EXPECT_EQ(solution.reverseWords(s), expected);
+}
+
+TEST(Problem151, Case4) {
+  string s{"F R  I   E    N     D      S      "};
+  string expected{"S D N E I R F"};
+  Solution solution;
+  EXPECT_EQ(solution.reverseWords(s), expected);
+}