diff --git a/src/main/java/com/cool/hot100/backtracking/Num17.java b/src/main/java/com/cool/hot100/backtracking/Num17.java
new file mode 100644
index 0000000..b91ef59
--- /dev/null
+++ b/src/main/java/com/cool/hot100/backtracking/Num17.java
@@ -0,0 +1,66 @@
+package com.cool.hot100.backtracking;
+
+import java.util.*;
+
+/**
+ * Created with IntelliJ IDEA.
+ *
+ * @Author: Cool
+ * @Date: 2024/09/10/11:06
+ * DayNumber 1
+ * Hard 2
+ * Level ?
+ */
+public class Num17 {
+
+    List<String> res = new ArrayList<>();
+    char[] chars;
+
+    public List<String> letterCombinations(String digits) {
+        if ("".equals(digits)) {
+            return new ArrayList<>();
+        }
+        chars = new char[digits.length()];
+        dfs(0, digits);
+        return new ArrayList<>(res);
+    }
+
+    private void dfs(int index, String digits) {
+        if (index >= digits.length()) {
+            res.add(new String(chars));
+            return;
+        }
+        String str = getLetter(digits.charAt(index)-'0');
+
+        for (int i = 0; i < str.length(); i++) {
+            chars[index] = str.charAt(i);
+            dfs(index + 1, digits);
+
+        }
+
+    }
+
+    private String getLetter(int num) {
+        switch (num) {
+            case 2:
+                return "abc";
+            case 3:
+                return "def";
+            case 4:
+                return "ghi";
+            case 5:
+                return "jkl";
+            case 6:
+                return "mno";
+            case 7:
+                return "pqrs";
+            case 8:
+                return "tuv";
+            case 9:
+                return "wxyz";
+            default:
+                return "";
+        }
+    }
+
+}
diff --git a/src/main/java/com/cool/hot100/backtracking/Num22.java b/src/main/java/com/cool/hot100/backtracking/Num22.java
new file mode 100644
index 0000000..46ce298
--- /dev/null
+++ b/src/main/java/com/cool/hot100/backtracking/Num22.java
@@ -0,0 +1,43 @@
+package com.cool.hot100.backtracking;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Created with IntelliJ IDEA.
+ *
+ * @Author: Cool
+ * @Date: 2024/09/10/15:25
+ * DayNumber 3
+ * Hard 2
+ * Level ?
+ */
+public class Num22 {
+
+    int n;
+    char[] chars;
+    List<String> list = new ArrayList<>();
+
+    public List<String> generateParenthesis(int n) {
+        this.n = n;
+        chars = new char[n * 2];
+        dfs(0, 0);
+        return list;
+    }
+
+    private void dfs(int num, int left) {
+        if (num == n * 2) {
+            list.add(new String(chars));
+            return;
+        }
+        if (left < n) {
+            chars[num] = '(';
+            dfs(num + 1, left + 1);
+        }
+        if (left * 2 > num) {
+            chars[num] = ')';
+            dfs(num + 1, left);
+        }
+
+    }
+}
diff --git a/src/main/java/com/cool/hot100/backtracking/Num39.java b/src/main/java/com/cool/hot100/backtracking/Num39.java
new file mode 100644
index 0000000..1b789da
--- /dev/null
+++ b/src/main/java/com/cool/hot100/backtracking/Num39.java
@@ -0,0 +1,56 @@
+package com.cool.hot100.backtracking;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * Created with IntelliJ IDEA.
+ *
+ * @Author: Cool
+ * @Date: 2024/09/10/14:21
+ * DayNumber 2
+ * Hard 2
+ * Level ?
+ */
+public class Num39 {
+
+    List<List<Integer>> resList = new ArrayList<>();
+    int maxIndex;
+    List<Integer> list = new ArrayList<>();
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        Arrays.sort(candidates);
+        for (int i = 0; i < candidates.length; i++) {
+            if (candidates[i] > target) {
+                maxIndex = i - 1;
+                break;
+            } else if (i == candidates.length - 1) {
+                maxIndex = i;
+            }
+        }
+        if (candidates[0] > target || maxIndex < 0) {
+            return resList;
+        }
+        dfs(candidates, target, maxIndex);
+        return resList;
+    }
+
+    private void dfs(int[] candidates, int target, int index) {
+        if (target == 0) {
+            resList.add(new ArrayList<>(list));
+            return;
+        }
+        if (target < 0 || index < 0) {
+            return;
+        }
+        if (candidates[index] <= target) {
+            list.add(candidates[index]);
+            dfs(candidates, target - candidates[index], index);
+            list.remove(list.size() - 1);
+        }
+        dfs(candidates, target, index - 1);
+    }
+
+
+}