Optimize- get third largest num in array

Question

So, I was working on this challenge to return the third largest number in an array. I had got it worked out until I realized that I must account for repeat numbers. I handled this by adding 3 layers of for loops with variables i, j, and k. You'll see what I mean in the code. This is not terribly efficient or scalable.

My question is, how can I optimize this code? What other methods should I be using?

function thirdGreatest (arr) {
    arr.sort(function(a, b) {
        if (a < b) {
            return 1;
        } else if (a > b) {
            return -1;
        } else {
            return 0;
        }
    });

    for ( var i = 0; i < arr.length; i++) {
        for (var j = 1; j < arr.length; j++) {
            for (var k = 2; k < arr.length; k++) {
                if (arr[i] > arr[j]) {
                    if (arr[j] > arr[k]) {
                        return arr[k];
                    }

                }
            }
        }
    }


}

console.log(thirdGreatest([5, 3, 23, 7,3,2,5,10,24,2,31, 31, 31])); // 23
console.log(thirdGreatest([5, 3, 23, 7,3,2,5,10,24,2,31])) // 23
console.log(thirdGreatest([5, 3, 7, 4])); // 4
console.log(thirdGreatest([2, 3, 7, 4])); // 3

Answer 1

Since you already sorted the array, it seems like you should be fine iterating over the list and keep track of the numbers you have already seen. When you have seen three different numbers, return the current one:

var seen = [arr[0]];

for (var i = 1; i < arr.length; i++) {
  if (arr[i] !== seen[0]) {
    if (seen.length === 2) {
      return arr[i];
    }
    seen.unshift(arr[i]);
  }
}

function thirdGreatest (arr) {
    arr.sort(function(a, b) {
        return b - a;
    });

    var seen = [arr[0]];
    
    for (var i = 1; i < arr.length; i++) {
      if (arr[i] !== seen[0]) {
        if (seen.length === 2) {
          return arr[i];
        }
        seen.unshift(arr[i]);
      }
    }
}

console.log(thirdGreatest([5, 3, 23, 7,3,2,5,10,24,2,31, 31, 31])); // 23
console.log(thirdGreatest([5, 3, 23, 7,3,2,5,10,24,2,31])) // 23
console.log(thirdGreatest([5, 3, 7, 4])); // 4
console.log(thirdGreatest([2, 3, 7, 4])); // 3

---

Note: You can simplify the sort callback to

arr.sort(function(a, b) {
  return b - a;
});
// With arrow functions:
// arr.sort((a, b) => b - a);

The callback has to return a number that is larger, smaller or equal to 0, it doesn't have to be exactly -1 or 1.

Answer 2

A one-"line"r using Set to remove duplicates

Array.from(new Set(arr)).sort(function(a, b) {
    return b - a;
})[2];

Set now has reasonable browser support