diff --git a/README.md b/README.md
index 6e976e4..730fe32 100644
--- a/README.md
+++ b/README.md
@@ -140,3 +140,11 @@ eg: [1,3,4,2,5]<br />
 2的小数:1<br />
 5的小数:1,3,4,2<br />
 小数和为:1+1+3+1+1+3+4+2
+
+### 前缀树
+1）单个字符串中，字符从前到后的加到一棵多叉树上 <br />
+2）字符放在边上，节点上有专属的数据项（常见的是pass和end值）<br />
+3）样本添加方式，每个字符串都从根节点开始加，如果没有路就新建，如果有路就复用<br />
+4）添加时，沿途节点的pass值加1，每个字符串结束时来到的节点end值加1<br />
+
+end值可查某个字符串出现几次,pass值可查以某个字符串为前缀的字符串出现过几次
diff --git a/src/algorithm/NetherlandsFlag.java b/src/algorithm/NetherlandsFlag.java
new file mode 100644
index 0000000..35c8134
--- /dev/null
+++ b/src/algorithm/NetherlandsFlag.java
@@ -0,0 +1,57 @@
+package algorithm;
+
+/**
+ * 荷兰国旗问题的划分
+ * arr[L....R] 以arr[R]作为划分值
+ * <arr[R]的在左侧 =arr[R]的在中间 >arr[R]的在右侧
+ *
+ * @author Wen
+ * @date 2022/8/8 22:05
+ */
+public class NetherlandsFlag {
+
+    //返回划分过后=arr[R]的数的左边界和右边界
+    public int[] cal(int[] arr, int L, int R) {
+        if (L > R) {
+            return new int[]{-1, -1};
+        }
+        if (L == R) {
+            return new int[]{L, R};
+        }
+        int index = 0;
+        int less = 0;
+        int large = R;
+        int num = arr[R];
+        while (index <= large) {
+            if (arr[index] == num) {
+                index++;
+                continue;
+            }
+            if (arr[index] < num) {
+                int temp = arr[less];
+                arr[less] = arr[index];
+                arr[index] = temp;
+                less++;
+                index++;
+                continue;
+            }
+            if (arr[index] > num) {
+                int temp = arr[large];
+                arr[large] = arr[index];
+                arr[index] = temp;
+                large--;
+            }
+        }
+        return new int[]{less , large};
+    }
+
+    public static void main(String[] args) {
+        NetherlandsFlag netherlandsFlag = new NetherlandsFlag();
+        int[] source = new int[]{1, 3, 5, 6, 3, 4, 3, 4, 8, 3};
+        netherlandsFlag.cal(source, 0, 9);
+        for (int i : source) {
+            System.out.println(i);
+        }
+    }
+
+}
diff --git a/src/ds/tree/PrefixTree.java b/src/ds/tree/PrefixTree.java
new file mode 100644
index 0000000..fd31593
--- /dev/null
+++ b/src/ds/tree/PrefixTree.java
@@ -0,0 +1,109 @@
+package ds.tree;
+
+/**
+ * @author Wen
+ * @date 2022/8/22 22:26
+ */
+public class PrefixTree {
+    private Node head;
+
+    public PrefixTree() {
+        head = new Node();
+    }
+
+    //将字符串插入前缀树中
+    public void insert(String str) {
+        if (str == null) {
+            return;
+        }
+        Node node = head;
+        node.pass++;
+        char[] chars = str.toCharArray();
+        for (char aChar : chars) {
+            int index = aChar - 'a';//在哪个节点上
+            if (node.next[index] == null) {
+                node.next[index] = new Node();
+            }
+            node.next[index].pass++;
+            node = node.next[index];
+        }
+        node.end++;
+    }
+
+    //str出现过几次
+    public Integer search(String str) {
+        if (str == null) {
+            return 0;
+        }
+        Node node = head;
+        char[] chars = str.toCharArray();
+        for (char aChar : chars) {
+            int index = aChar - 'a';//在哪个节点上
+            if (node.next[index] == null) {
+                return 0;
+            }
+            node = node.next[index];
+        }
+        return node.end;
+    }
+
+    public void delete(String str) {
+        if (search(str) > 0) {
+            Node node = head;
+            node.pass--;
+            char[] chars = str.toCharArray();
+            for (char aChar : chars) {
+                int index = aChar - 'a';//在哪个节点上
+                if (--node.next[index].pass == 0) {//pass为0后，后面节点直接丢弃
+                    node.next[index] = null;
+                    return;
+                }
+                node = node.next[index];
+            }
+            node.end--;
+        }
+    }
+
+    //以prefix为前缀的字符串个数
+    public Integer prefixNumber(String prefix) {
+        if (prefix == null) {
+            return 0;
+        }
+        Node node = head;
+        char[] chars = prefix.toCharArray();
+        for (char aChar : chars) {
+            int index = aChar - 'a';//在哪个节点上
+            if (node.next[index] == null) {
+                return 0;
+            }
+            node = node.next[index];
+        }
+        return node.pass;
+    }
+
+    class Node {
+        int pass;
+        int end;
+        Node[] next;
+
+        public Node() {
+            this.pass = 0;
+            this.end = 0;
+            next = new Node[26];//假设字符串都是26个小写字母
+        }
+    }
+
+    public static void main(String[] args) {
+        PrefixTree prefixTree = new PrefixTree();
+        prefixTree.insert("abc");
+        prefixTree.insert("abc");
+        prefixTree.insert("eewd");
+        prefixTree.insert("adb");
+        prefixTree.insert("eewd");
+        prefixTree.insert("abs");
+        prefixTree.insert("eewd");
+        System.out.println(prefixTree.search("abc"));
+        System.out.println(prefixTree.search("eewd"));
+        System.out.println(prefixTree.prefixNumber("ab"));
+    }
+}