diff --git a/src/66.plus-one.c b/src/66.plus-one.c
new file mode 100644
index 0000000..7b4f466
--- /dev/null
+++ b/src/66.plus-one.c
@@ -0,0 +1,96 @@
+/** 66. Plus One */
+
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int *plusOne(int *digits, int digitsSize, int *returnSize) {
+  int i = 0;
+  int *newDigits = calloc(digitsSize + 1, sizeof(int));
+  for (i = 0; i < digitsSize; i++) {
+    newDigits[i+1] = digits[i];
+    assert(newDigits[i+1] < 10 && newDigits[i+1] >= 0);
+  }
+  i = digitsSize;
+  do {
+      newDigits[i]++;
+      if (newDigits[i] == 10) {
+          newDigits[i] = 0;
+          i--;
+      } else {
+          break;
+      }
+  } while (i >= 0);
+  if (i == 0) {
+    *returnSize = digitsSize + 1;
+  } else {
+    newDigits++;
+    *returnSize = digitsSize;
+  }
+  return newDigits;
+}
+
+bool equal(int *nums1, int *nums2, int len) {
+  for (int i = 0; i < len; i++) {
+    if (nums1[i] != nums2[i])
+      return false;
+  }
+  return true;
+}
+
+void printDigits(int *digits, int digitsSize) {
+  for (int i = 0; i < digitsSize; i++) {
+    printf("%d", digits[i]);
+  }
+  printf("\n");
+}
+
+void test_1() {
+  int nums[] = {1, 9};
+  int expected[] = {2, 0};
+  int len = 0;
+  int *result = plusOne(nums, 2, &len);
+  printDigits(result, len);
+  assert(len == 2);
+  assert(equal(result, expected, len));
+}
+
+
+void test_2() {
+  int *nums = malloc(0 * sizeof(int));
+  int expected[] = {1};
+  int len = 0;
+  int *result = plusOne(nums, 0, &len);
+  printDigits(result, len);
+  assert(len == 1);
+  assert(equal(result, expected, len));
+}
+
+void test_3() {
+  int nums[] = {1, 9, 9};
+  int expected[] = {2, 0, 0};
+  int len = 0;
+  int *result = plusOne(nums, 3, &len);
+  printDigits(result, len);
+  assert(len == 3);
+  assert(equal(result, expected, len));
+}
+
+void test_4() {
+  int nums[] = {9};
+  int expected[] = {1, 0};
+  int len = 0;
+  int *result = plusOne(nums, 1, &len);
+  printDigits(result, len);
+  assert(len == 2);
+  assert(equal(result, expected, len));
+}
+
+int main() {
+    test_1();
+    test_2();
+    test_3();
+    test_4();
+
+}