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JavaScript

Posted by Yuval Blass
Yuval Blass
18,134 Points

CodeFights challenge

Hi everyone, I tried to solve this (https://codefights.com/interview-practice/task/pMvymcahZ8dY4g75q/) challenge, and I have no idea why my code isn't working. I tried to debug it with console.log function, and I have seen something weird; Why the first console.log(nArray) isn't the same as to the seconde one?

function firstDuplicate(a) {
    const nArray = a;
    console.log(nArray);
    var sorted_arr = a.sort();
    console.log(nArray);
    var results = [];
    for (var i = 0; i < a.length - 1; i++) {
        if (sorted_arr[i + 1] == sorted_arr[i]) {
            results.push(sorted_arr[i]);
        }
    }
    if (results.length == 1) return results[0];
    if (results.length > 1) {
        var index = nArray.lastIndexOf(results[0]);
        var num = results[0];
        for (var j = 1; j < results.length; j++) {
            if (nArray.lastIndexOf(results[j]) < index) {
                num = results[j];
                index = nArray.lastIndexOf(results[j]);
            }
        }
        return num;
    }
    return -1;    
}

firstDuplicate([2, 3, 3, 1, 5, 2]);

Thanks in advanced, Yuval Blass.

Steven Parker
Steven Parker
231,007 Points
Steven Parker
Steven Parker
231,007 Points

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2 Answers

Steven Parker
Steven Parker
231,007 Points
Steven Parker
Steven Parker
231,007 Points

The log outputs differ because the array gets sorted between them.

In an assignment of an array, the array is not copied but the new variable becomes a reference to the same array. So when "a.sort()" is performed between the two log functions, the array that nArray refers to is changed.

If you want to copy an array instead of creating a new reference to it, a common technique is to use the slice function with no argument:

    const nArray = a.slice();
Yuval Blass
Yuval Blass
18,134 Points
Yuval Blass
Yuval Blass
18,134 Points

Hi Steven,

How can I create a variable without a reference to the array? How can I make the first four lines works as I want them to be?

Here is the challenge description:

Note: Write a solution with O(n) time complexity and O(1) additional space complexity, since this is what you would be asked to do during a real interview.

Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more than 1 duplicated numbers, return the number for which the second occurrence has a smaller index than the second occurrence of the other number does. If there are no such elements, return -1.

Example

For a = [2, 3, 3, 1, 5, 2], the output should be firstDuplicate(a) = 3.

There are 2 duplicates: numbers 2 and 3. The second occurrence of 3 has a smaller index than than second occurrence of 2 does, so the answer is 3.

For a = [2, 4, 3, 5, 1], the output should be firstDuplicate(a) = -1.

Thanks, Yuval Blass.

Steven Parker
Steven Parker
231,007 Points
Steven Parker
Steven Parker
231,007 Points

I updated my answer to cover this question also.

In future questions, when posting about a challenge it's helpful to provide a link to the challenge page.