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Detecting the first non-unique element in array in JavaScript
We are required to write a function that returns the index of the very first element that appears at least twice in the array. If no element appears more than once, we have to return -1. We have to do this in constant space (i.e., without utilizing extra memory).
So, let's write the solution for this problem.
We will use a for loop to iterate over the array and use the Array.prototype.lastIndexOf() method to check for duplicacy.
Example
The code for this will be −
const arr1 = [0, 1, 1, 2, 3, 4, 4, 5]; const firstRedundant = arr => { for(let i = 0; i < arr.length; i++){ if(arr.lastIndexOf(arr[i]) !== i){ return i; }; }; return -1; } console.log(firstRedundant(arr1)); // 1
Output
The output in the console will be −
1
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