diff --git a/Week_04/id_38/LeetCode_169_038.java b/Week_04/id_38/LeetCode_169_038.java
new file mode 100644
index 00000000..c556f19b
--- /dev/null
+++ b/Week_04/id_38/LeetCode_169_038.java
@@ -0,0 +1,32 @@
+/*
+ * @lc app=leetcode.cn id=169 lang=java
+ *
+ * [169] 求众数
+ */
+
+class LeetCode_169_038 {
+
+  public int majorityElement(int[] nums) {
+    Arrays.sort(nums);
+    return nums[nums.length/2];
+  }
+
+  public int majorityElement2(int[] nums) {
+    int majorityCount = nums.length / 2;
+
+    Map<Integer, Integer> countMap = new HashMap();
+    for (int num : nums) {
+      if (countMap.get(num) != null) {
+        countMap.put(num, countMap.get(num) + 1);
+      } else {
+        countMap.put(num, 1);
+      }
+
+      if (countMap.get(num) > majorityCount) {
+        return num;
+      }
+    }
+
+    return -1;
+  }
+}
diff --git a/Week_04/id_38/LeetCode_455_038.java b/Week_04/id_38/LeetCode_455_038.java
new file mode 100644
index 00000000..57684879
--- /dev/null
+++ b/Week_04/id_38/LeetCode_455_038.java
@@ -0,0 +1,37 @@
+/*
+ * @lc app=leetcode.cn id=455 lang=java
+ *
+ * [455] 分发饼干
+ */
+class LeetCode_455_038 {
+  public int findContentChildren(int[] g, int[] s) {
+    Arrays.sort(g);
+    Arrays.sort(s);
+    int count = 0;
+    int i = 0, j = 0;
+    while (i < g.length && j < s.length) {
+      if (g[i] <= s[j]) {
+        i++;
+        j++;
+        count++;
+      } else {
+        j++;
+      }
+    }
+    return count;
+  }
+
+  public int findContentChildren2(int[] g, int[] s) {
+    Arrays.sort(g);
+    Arrays.sort(s);
+    int i = 0, j = 0;
+    int l1 = g.length, l2 = s.length;
+    while (i < l1 && j < l2) {
+      if (g[i] <= s[j]) {
+        i++;
+      }
+      j++;
+    }
+    return i;
+  }
+}
diff --git a/Week_04/id_38/LeetCode_51_038.java b/Week_04/id_38/LeetCode_51_038.java
new file mode 100644
index 00000000..256aa8cc
--- /dev/null
+++ b/Week_04/id_38/LeetCode_51_038.java
@@ -0,0 +1,76 @@
+/*
+ * @lc app=leetcode.cn id=51 lang=java
+ *
+ * [51] N皇后
+ */
+
+class LeetCode_51_038 {
+
+    int rows[];
+    // "hill" diagonals
+    int hills[];
+    // "dale" diagonals
+    int dales[];
+    int n;
+    // output
+    List<List<String>> output = new ArrayList();
+    // queens positions
+    int queens[];
+  
+    public boolean isNotUnderAttack(int row, int col) {
+      int res = rows[col] + hills[row - col + 2 * n] + dales[row + col];
+      return (res == 0) ? true : false;
+    }
+  
+    public void placeQueen(int row, int col) {
+      queens[row] = col;
+      rows[col] = 1;
+      hills[row - col + 2 * n] = 1;  // "hill" diagonals
+      dales[row + col] = 1;   //"dale" diagonals
+    }
+  
+    public void removeQueen(int row, int col) {
+      queens[row] = 0;
+      rows[col] = 0;
+      hills[row - col + 2 * n] = 0;
+      dales[row + col] = 0;
+    }
+  
+    public void addSolution() {
+      List<String> solution = new ArrayList<String>();
+      for (int i = 0; i < n; ++i) {
+        int col = queens[i];
+        StringBuilder sb = new StringBuilder();
+        for(int j = 0; j < col; ++j) sb.append(".");
+        sb.append("Q");
+        for(int j = 0; j < n - col - 1; ++j) sb.append(".");
+        solution.add(sb.toString());
+      }
+      output.add(solution);
+    }
+  
+    public void backtrack(int row) {
+      for (int col = 0; col < n; col++) {
+        if (isNotUnderAttack(row, col)) {
+          placeQueen(row, col);
+          // if n queens are already placed
+          if (row + 1 == n) addSolution();
+            // if not proceed to place the rest
+          else backtrack(row + 1);
+          // backtrack
+          removeQueen(row, col);
+        }
+      }
+    }
+  
+    public List<List<String>> solveNQueens(int n) {
+      this.n = n;
+      rows = new int[n];
+      hills = new int[4 * n - 1];
+      dales = new int[2 * n - 1];
+      queens = new int[n];
+  
+      backtrack(0);
+      return output;
+    }
+}
diff --git a/Week_04/id_38/LeetCode_70_038.java b/Week_04/id_38/LeetCode_70_038.java
new file mode 100644
index 00000000..4a6997c9
--- /dev/null
+++ b/Week_04/id_38/LeetCode_70_038.java
@@ -0,0 +1,35 @@
+/*
+ * @lc app=leetcode.cn id=70 lang=java
+ *
+ * [70] 爬楼梯
+ */
+
+class LeetCode_70_038 {
+
+    public int climbStairs(int n) {
+        if (n == 1){
+            return 1;
+        }
+        int[] dp = new int[n + 1];
+        dp[1] = 1;
+        dp[2] = 2;
+        for (int i = 3; i <= n; i++){
+            dp[i] = dp[i -1] + dp[i - 2];
+        }
+        return dp[n];
+    }
+
+    public int climbStairs2(int n) {
+        return climb_Stairs(0, n);
+    }
+    
+    public int climb_Stairs(int i, int n) {
+        if (i > n) {
+            return 0;
+        }
+        if (i == n) {
+            return 1;
+        }
+        return climb_Stairs(i + 1, n) + climb_Stairs(i + 2, n);
+    }
+}
diff --git a/Week_04/id_38/LeetCode_720_038.java b/Week_04/id_38/LeetCode_720_038.java
new file mode 100644
index 00000000..6bb3af5e
--- /dev/null
+++ b/Week_04/id_38/LeetCode_720_038.java
@@ -0,0 +1,91 @@
+/*
+ * @lc app=leetcode.cn id=720 lang=java
+ *
+ * [720] 词典中最长的单词
+ */
+
+class LeetCode_720_038 {
+
+    public String longestWord(String[] words) {
+        Set<String> s = new HashSet<>();
+        for(String word : words)
+            s.add(word);
+        int ml = 0;
+        String mw = "";
+        for(String w : words){
+            if(w.length() < ml) continue;
+            if(w.length() == ml && w.compareTo(mw) >= 0) continue;
+            int n = w.length();
+            boolean flag = true;
+            for(int i = 1; i < n; i++){
+                if(!s.contains(w.substring(0, i))){
+                    flag = false;
+                    break;
+                }   
+            }
+            if(flag){
+                ml = w.length();
+                mw = w;
+            }    
+        }
+        
+        return mw;
+    }
+
+
+    //#endregion
+    private static final int R = 26;
+    private TrieNode root;
+
+    private static class TrieNode {
+        private final TrieNode[] children;
+        private String word;
+
+        private TrieNode() {
+            children = new TrieNode[R];
+            word = null;
+        }
+    }
+
+    private void insertWord(String word) {
+        int len = word.length();
+        TrieNode curr = root;
+        char c;
+        int idx;
+        for (int i = 0; i < len; ++i) {
+            c = word.charAt(i);
+            idx = c - 'a';
+
+            if (curr.children[idx] == null) {
+                curr.children[idx] = new TrieNode();
+            }
+
+            curr = curr.children[idx];
+        }
+
+        curr.word = word;
+    }
+
+    private String findLongestWord(TrieNode root) {
+        String rst = root.word == null ? "" : root.word;
+        for (TrieNode child : root.children) {
+            if (child == null || child.word == null) continue;
+            String s = findLongestWord(child);
+            if (s.length() > rst.length()) rst = s;
+        }
+        return rst;
+    }
+
+    public String longestWord2(String[] words) {
+        // initialize a new trie
+        root = new TrieNode();
+
+        // construct the trie based on input dictionary words
+        for (String word : words) {
+            insertWord(word);
+        }
+
+        // find the longest dictionary word in this trie according to given rules
+        return findLongestWord(root);
+    }
+}
diff --git a/Week_04/id_38/LeetCode_784_038.java b/Week_04/id_38/LeetCode_784_038.java
new file mode 100644
index 00000000..0c81d770
--- /dev/null
+++ b/Week_04/id_38/LeetCode_784_038.java
@@ -0,0 +1,32 @@
+/*
+ * @lc app=leetcode.cn id=784 lang=java
+ *
+ * [784] 字母大小写全排列
+ */
+
+class LeetCode_784_038 {
+  public List<String> letterCasePermutation(String S) {
+    List<String> ans = new ArrayList<>();
+    StringBuilder root = new StringBuilder(S);
+    dfs(ans, root, 0);
+    return ans;
+  }
+
+  private void dfs(List<String> ans, StringBuilder root, int index) {
+    if (index == root.length()) {
+      ans.add(root.toString());
+      return;
+    }
+    char c = root.charAt(index);
+    if ('a' <= c && c <= 'z') {
+      root.setCharAt(index, (char) (c - 32));
+      dfs(ans, root, index + 1);
+      root.setCharAt(index, c);
+    } else if ('A' <= c && c <= 'Z') {
+      root.setCharAt(index, (char) (c + 32));
+      dfs(ans, root, index + 1);
+      root.setCharAt(index, c);
+    }
+    dfs(ans, root, index + 1);
+  }
+}