Contains Duplicate and Missing Number
Too Long; Didn't Read
Problem solved by Sergei Golitsyn: “Given an integer array `nums` returns `true` if any value appears at least twice in the array, and return `false`’s if every element is distinct in the array. Solution: We should find the missing number. From the description, we know that all elements are unique. And for example, if we have arr length three, there must be 3 elements. If we find a sum for one of the elements from 1 to 3 to 3, we will have an array like \[1,2,3,1]