diff --git a/include/s0076_minimum_window_substring.hpp b/include/s0076_minimum_window_substring.hpp
new file mode 100644
index 0000000..f085698
--- /dev/null
+++ b/include/s0076_minimum_window_substring.hpp
@@ -0,0 +1,16 @@
+#ifndef S0076_MINIMUM_WINDOW_SUBSTRING_HPP
+#define S0076_MINIMUM_WINDOW_SUBSTRING_HPP
+
+#include <string>
+#include <unordered_map>
+
+using namespace std;
+
+class Solution {
+ public:
+  string minWindow(string s, string t);
+  unordered_map<char, int> ori, cnt;
+  bool check();
+};
+
+#endif
diff --git a/include/s0209_minimum_size_subarray_sum.hpp b/include/s0209_minimum_size_subarray_sum.hpp
new file mode 100644
index 0000000..9477e10
--- /dev/null
+++ b/include/s0209_minimum_size_subarray_sum.hpp
@@ -0,0 +1,14 @@
+#ifndef S0209_MINIMUM_SIZE_SUBARRAY_SUM_HPP
+#define S0209_MINIMUM_SIZE_SUBARRAY_SUM_HPP
+
+#include <vector>
+#include <climits>
+
+using namespace std;
+
+class Solution {
+ public:
+  int minSubArrayLen(int target, vector<int>& nums);
+};
+
+#endif
diff --git a/notes/src/SUMMARY.md b/notes/src/SUMMARY.md
index 50acb15..af4619f 100644
--- a/notes/src/SUMMARY.md
+++ b/notes/src/SUMMARY.md
@@ -2,6 +2,7 @@
 
 - [二分查找](./bin_search.md)
 - [移除元素](./remove_elements.md)
+- [长度最小的子数组](./minimum_size_subarray_sum.md)
 
 # 经典代码
 
diff --git a/notes/src/minimum_size_subarray_sum.md b/notes/src/minimum_size_subarray_sum.md
new file mode 100644
index 0000000..131d70d
--- /dev/null
+++ b/notes/src/minimum_size_subarray_sum.md
@@ -0,0 +1,20 @@
+# 长度最小的子数组
+
+当题目中含有：
+
+- 数组
+- 连续子数组
+
+这两个关键词时，考虑使用非定长滑动窗口（即长度不固定的滑动窗口）
+
+这种滑动窗口有两个关键：
+
+- 起始指针，什么条件下移动
+- 终止指针，什么条件下移动
+
+想清楚这两个问题就可以解答了。
+
+## 相关题目
+
+- [209. 长度最小的子数组](https://leetcode.com/problems/minimum-size-subarray-sum/)
+- [76. 最小覆盖子串](https://leetcode.com/problems/minimum-window-substring/)
diff --git a/src/s0076_minimum_window_substring.cpp b/src/s0076_minimum_window_substring.cpp
new file mode 100644
index 0000000..355a10d
--- /dev/null
+++ b/src/s0076_minimum_window_substring.cpp
@@ -0,0 +1,39 @@
+#include "s0076_minimum_window_substring.hpp"
+
+bool Solution::check() {
+  for (const auto &p : ori) {
+    if (cnt[p.first] < p.second) {
+      return false;
+    }
+  }
+  return true;
+}
+
+// 不包含 t 中所有字符，r 右移
+// 包含 t 中所有字符，l 右移
+string Solution::minWindow(string s, string t) {
+  for (const auto &c : t) {
+    ++ori[c];
+  }
+
+  int l = 0, r = -1;
+  int len = INT_MAX, ansL = -1, ansR = -1;
+
+  while (r < int(s.size())) {
+    if (ori.find(s[++r]) != ori.end()) {
+      ++cnt[s[r]];
+    }
+    while (check() && l <= r) {
+      if (r - l + 1 < len) {
+        len = r - l + 1;
+        ansL = l;
+      }
+      if (ori.find(s[l]) != ori.end()) {
+        --cnt[s[l]];
+      }
+      ++l;
+    }
+  }
+
+  return ansL == -1 ? string() : s.substr(ansL, len);
+}
diff --git a/src/s0209_minimum_size_subarray_sum.cpp b/src/s0209_minimum_size_subarray_sum.cpp
new file mode 100644
index 0000000..ef0195c
--- /dev/null
+++ b/src/s0209_minimum_size_subarray_sum.cpp
@@ -0,0 +1,32 @@
+#include "s0209_minimum_size_subarray_sum.hpp"
+
+int Solution::minSubArrayLen(int target, vector<int>& nums) {
+  int len = nums.size();
+  int result{INT_MAX};
+  for (int begin{0}, end{0}, sum{0}; end < len; ++end) {
+    sum += nums[end];
+    // 当滑动窗口里的值的总和大于等于 target 时，移动 begin 直到 sum 比 target
+    // 小
+    if (sum >= target) {
+      while (begin < end && sum >= target) {
+        sum -= nums[begin];
+        ++begin;
+      }
+      // 循环结束后有以下几种情况
+      // 1. begin == end && sum >= target 此时 result == 1
+      // 2. begin == end && sum < target 此时 result == 2
+      // 3. begin < end && sum < target 此时 result == end - begin + 2
+      int tmp;
+      if (begin == end) {
+        tmp = sum >= target ? 1 : 2;
+      } else {
+        tmp = end - begin + 2;
+      }
+      // 更新 result
+      result = tmp < result ? tmp : result;
+    }
+  }
+  // 如果 result == INT_MAX 说明 result
+  // 从来没更新过，也就意味着即使所有数组元素加在一起也比 target 小，此时返回 0
+  return result == INT_MAX ? 0 : result;
+}
diff --git a/tests/s0076_minimum_window_substring.cpp b/tests/s0076_minimum_window_substring.cpp
new file mode 100644
index 0000000..08ddc53
--- /dev/null
+++ b/tests/s0076_minimum_window_substring.cpp
@@ -0,0 +1,27 @@
+#include "s0076_minimum_window_substring.hpp"
+
+#include <gtest/gtest.h>
+
+TEST(Problem76, Case1) {
+  string s{"ADOBECODEBANC"};
+  string t{"ABC"};
+  string expected{"BANC"};
+  Solution solution;
+  EXPECT_EQ(solution.minWindow(s, t), expected);
+}
+
+TEST(Problem76, Case2) {
+  string s{"a"};
+  string t{"a"};
+  string expected{"a"};
+  Solution solution;
+  EXPECT_EQ(solution.minWindow(s, t), expected);
+}
+
+TEST(Problem76, Case3) {
+  string s{"a"};
+  string t{"aa"};
+  string expected{""};
+  Solution solution;
+  EXPECT_EQ(solution.minWindow(s, t), expected);
+}
diff --git a/tests/s0209_minimum_size_subarray_sum.cpp b/tests/s0209_minimum_size_subarray_sum.cpp
new file mode 100644
index 0000000..edeb553
--- /dev/null
+++ b/tests/s0209_minimum_size_subarray_sum.cpp
@@ -0,0 +1,35 @@
+#include "s0209_minimum_size_subarray_sum.hpp"
+
+#include <gtest/gtest.h>
+
+TEST(Problem209, Case1) {
+  int target{7};
+  vector<int> nums{2, 3, 1, 2, 4, 3};
+  int o{2};
+  Solution solution;
+  EXPECT_EQ(solution.minSubArrayLen(target, nums), o);
+}
+
+TEST(Problem209, Case2) {
+  int target{4};
+  vector<int> nums{1, 4, 4};
+  int o{1};
+  Solution solution;
+  EXPECT_EQ(solution.minSubArrayLen(target, nums), o);
+}
+
+TEST(Problem209, Case3) {
+  int target{11};
+  vector<int> nums{1, 1, 1, 1, 1, 1, 1, 1};
+  int o{0};
+  Solution solution;
+  EXPECT_EQ(solution.minSubArrayLen(target, nums), o);
+}
+
+TEST(Problem209, Case4) {
+  int target{11};
+  vector<int> nums{1, 2, 3, 4, 5};
+  int o{3};
+  Solution solution;
+  EXPECT_EQ(solution.minSubArrayLen(target, nums), o);
+}