Revert "Board: 简化 = 重载符函数"

This reverts commit be8b1d4baa.

src/game/board.cpp

@@ -45,9 +45,20 @@ Board::~Board()
 
 Board &Board::operator= (const Board &other)
 {
+    if (this == &other)
+        return *this;
+
     memcpy(this->locations, other.locations, sizeof(this->locations));
 
-    // TODO: 确定 millList 确实不用复制?
+    if (!millList.empty()) {
+        millList.clear();
+    }
+
+    if (!other.millList.empty()) {
+        for (auto i : other.millList) {
+            millList.push_back(i);
+        }
+    }
 
     return *this;
 }