adding solutions for 20-day's problems

solutions/039.md

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+### Typescript
+1720. Decode XORed Array
+
+```Typescript
+function decode(encoded: number[], first: number): number[] {
+    const arr:number[]=[first];
+    for(let i=0;i<encoded.length;i++){
+        let newItem = arr[i]^encoded[i]
+        arr.push(newItem)
+    }
+    return arr
+};
+```

solutions/040.md

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+### Typescript
+2657. Find the Prefix Common Array of Two Arrays
+
+```Typescript
+function findThePrefixCommonArray(A: number[], B: number[]): number[] {
+    const answer:number[]=[]
+    for(let i=0;i<A.length;i++){
+        let commonCount=0
+        for(let j=0;j<=i;j++){
+            if(A.slice(0,i+1).includes(B.slice(0,i+1)[j])){
+                commonCount+=1
+            }
+        }
+        answer.push(commonCount)
+    }
+    return answer
+};
+```