fix: kaifain /c/_ path issue

kaifain_v2/mixins/solution.ts

@@ -37,7 +37,7 @@ export const SolutionListMixin = Vue.extend({
       val = val !== '_' ? val : this.topCatId
 
       this.updateUrlPage(1, {
-        path: `/c/${val}`,
+        path: val !== '_' ? `/c/${val}` : '/',
         params: {
           cat_id: val
         },