Implement models/problem.js

+const mongoose=require('mongoose');
+
+const {Schema}=mongoose;
+
+const ProblemSchema=new Schema({
+    problemNum: {type: Number, required: true, unique: true},
+    problemTitle: {type: String, required: true},
+    solvedacLevel: {type: Number},
+    sumbitNum: {type: Number, required: true},
+    correctNum: {type: Number, required: true},
+    category: {type:[String]}
+});
+
+ProblemSchema.statics.findByProblemNum=function(problemNum){
+    return this.findOne({problemNum:problemNum});
+}
+
+ProblemSchema.methods.addCategory=function(category){
+    this.category.push(category);
+    return this.save();
+}
+
+ProblemSchema.methods.removeCategory=function(category){
+    const idx=this.category.findIndex(item=>item===category);
+    this.splice(idx,1);
+    return this.save();
+}
+
+ProblemSchema.methods.getProblemNum=function(){
+    return this.problemNum;
+}
+
+ProblemSchema.methods.getProblemTitle=function(){
+    return this.problemTitle;
+}
+
+ProblemSchema.methods.getSolvedacLevel=function(){
+    return this.solvedacLevel;
+}
+
+ProblemSchema.methods.getSumbitNum=function(){
+    return this.sumbitNum;
+}
+
+ProblemSchema.methods.getCorrectNum=function(){
+    return this.correctNum;
+}
+
+ProblemSchema.methods.getCategory=function(){
+    return this.category;
+}
+
+ProblemSchema.methods.serialize=function(){
+    return this.toJSON();
+}
+
+const Problem=mongoose.model('Problem',ProblemSchema);
+module.exports=Problem;
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