missing number with duplicates in the list : I have a list `l = [3,1,2,5,3]`. My goal is to find the repeated number in **O(n)** which is 3 and also the missing number which is 4. Finally, the output should be `[3,4]`. I trying to use a dictionary to find the repeated number but again to find the missing number I am using another loop which results in **O(n^2)** time complexity. Can anyone tell how to find the answer in **O(n)** time complexity?