Update README_EN.md

Author: yanglbme

solution/0000-0099/0017.Letter Combinations of a Phone Number/README_EN.md

@@ -553,35 +553,44 @@ class Solution {
 
 #### C
 
-```C
-char *map[] = {"",    "",    "abc",  "def", "ghi",
-               "jkl", "mno", "pqrs", "tuv", "wxyz"};
-
-void backtrack(char *d, int i, char *cur, char **res, int *sz) {
-  if (!d[i]) {
-    res[(*sz)++] = strdup(cur);
-    return;
-  }
-  for (char *p = map[d[i] - '0']; *p; p++) {
-    cur[i] = *p;
-    backtrack(d, i + 1, cur, res, sz);
-  }
-}
+```c
+char* d[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 
-char **letterCombinations(char *d, int *sz) {
-  *sz = 0;
-  if (!*d)
-    return NULL;
-  int max = 1, len = strlen(d);
-  for (int i = 0; i < len; i++)
-    max *= (d[i] == '7' || d[i] == '9') ? 4 : 3;
-  char **res = malloc(max * sizeof(char *)), *cur = malloc(len + 1);
-  cur[len] = '\0';
-  backtrack(d, 0, cur, res, sz);
-  free(cur);
-  return res;
-}
+char** letterCombinations(char* digits, int* returnSize) {
+    if (!*digits) {
+        *returnSize = 0;
+        return NULL;
+    }
 
+    int size = 1;
+    char** ans = (char**) malloc(sizeof(char*));
+    ans[0] = strdup("");
+
+    for (int x = 0; digits[x]; ++x) {
+        char* s = d[digits[x] - '2'];
+        int len = strlen(s);
+        char** t = (char**) malloc(sizeof(char*) * size * len);
+        int tSize = 0;
+
+        for (int i = 0; i < size; ++i) {
+            for (int j = 0; j < len; ++j) {
+                int oldLen = strlen(ans[i]);
+                char* tmp = (char*) malloc(oldLen + 2);
+                strcpy(tmp, ans[i]);
+                tmp[oldLen] = s[j];
+                tmp[oldLen + 1] = '\0';
+                t[tSize++] = tmp;
+            }
+            free(ans[i]);
+        }
+        free(ans);
+        ans = t;
+        size = tSize;
+    }
+
+    *returnSize = size;
+    return ans;
+}
 ```
 
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