diff --git a/src/22.c b/src/22.c
index 4975312..55abfd2 100644
--- a/src/22.c
+++ b/src/22.c
@@ -4,18 +4,93 @@
 #include <solution/22.h>
 #include <stdlib.h>
 #include <string.h>
-char **generateParenthesis(int n, int *returnSize)
+
+typedef struct tinode_t
 {
-    char **dp = malloc(sizeof(char *) * n);
-    for (int i = 0; i < n; i++)
+    char *ans;
+    struct tinode_t *left;
+    struct tinode_t *right;
+} tinode_t;
+
+int get_number_of(char *s, char c)
+{
+    int num = 0;
+    while (*s != '\0')
     {
-        dp[i] = malloc(sizeof(char) * (n + 1));
-        memset(dp[i], 0, n + 1);
+        if (*s == c)
+            num++;
+        s++;
+    }
+    return num;
+}
+
+void dfs(tinode_t *root, char ***ans, int n, int *retSize)
+{
+    if (root == NULL)
+        return;
+
+    // check current number of "("
+    int left = get_number_of(root->ans, '(');
+    // in these time ")" can not greater than "("
+    int right = get_number_of(root->ans, ')');
+    // we can add a "(" to the ans when "(" less than n
+    int s_len = strlen(root->ans);
+    if (left < n)
+    {
+        tinode_t *ln = malloc(sizeof(tinode_t));
+        ln->ans = malloc(s_len + 2);
+        strcpy(ln->ans, root->ans);
+        ln->ans[s_len] = '(';
+        ln->ans[s_len + 1] = '\0';
+        ln->left = NULL;
+        ln->right = NULL;
+        root->left = ln;
+    }
+    if (right < left)
+    {
+        tinode_t *rn = malloc(sizeof(tinode_t));
+        rn->ans = malloc(s_len + 2);
+        strcpy(rn->ans, root->ans);
+        rn->ans[s_len] = ')';
+        rn->ans[s_len + 1] = '\0';
+        rn->left = NULL;
+        rn->right = NULL;
+        root->right = rn;
+    }
+    // endpoint: left + right == 2n
+    if (right + left == 2 * n)
+    {
+        *ans = realloc(*ans, sizeof(char *) * (++(*retSize)));
+        // we assume realloc is always success
+        (*ans)[*retSize - 1] = malloc(2 * n + 1);
+        strcpy((*ans)[*retSize - 1], root->ans);
     }
 
-    for (int i = 0; i < n; i++)
-    {
-        free(dp[i]);
-    }
-    free(dp);
+    dfs(root->left, ans, n, retSize);
+    dfs(root->right, ans, n, retSize);
+}
+
+void free_tree(tinode_t *root)
+{
+    if (root == NULL)
+        return;
+    free_tree(root->left);
+    free_tree(root->right);
+    free(root->ans);
+    free(root);
+}
+
+char **generateParenthesis(int n, int *returnSize)
+{
+    char **result = NULL;
+    *returnSize = 0;
+    tinode_t *r = malloc(sizeof(tinode_t));
+    r->ans = malloc(2);
+    r->right = NULL;
+    r->left = NULL;
+    strcpy(r->ans, "(");
+    dfs(r, &result, n, returnSize);
+    free_tree(r);
+
+    return result;
 }
\ No newline at end of file