Refactor toHex function to accept int and use unsigned

Author: walkccc

solutions/405. Convert a Number to Hexadecimal/405.cpp

@@ -1,16 +1,17 @@
 class Solution {
  public:
-  string toHex(unsigned num) {
-    if (num == 0)
+  string toHex(int num) {
+    unsigned unsignedNum = static_cast<unsigned>(num);
+    if (unsignedNum == 0)
       return "0";
 
     constexpr char hex[] = {'0', '1', '2', '3', '4', '5', '6', '7',
                             '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
     string ans;
 
-    while (num != 0) {
-      ans += hex[num & 0xf];
-      num >>= 4;
+    while (unsignedNum != 0) {
+      ans += hex[unsignedNum & 0xf];
+      unsignedNum >>= 4;
     }
 
     ranges::reverse(ans);