Second Largest Element in Array (Java)

The problem

Given an array like {10, 5, 20, 8, 15}, the second largest element is 15 (the largest is 20). We can find it in a single pass using just two variables — no sorting needed. This is a common interview question.

The two-variable idea

Keep first (largest) and second (second largest), both starting at the smallest possible value. For each element:

• If it beats first, the old first becomes second, and this becomes first.
• Else if it beats second (but not first), it becomes second.

The Java program

public class SecondLargest {
    public static void main(String[] args) {
        int[] a = {10, 5, 20, 8, 15};
        int first = Integer.MIN_VALUE;
        int second = Integer.MIN_VALUE;

        for (int x : a) {
            if (x > first) {
                second = first;   // old max slides down
                first = x;
            } else if (x > second && x != first) {
                second = x;
            }
        }
        System.out.println("Second largest: " + second);
    }
}

Both start at Integer.MIN_VALUE. Each new maximum pushes the old one into second. Values between the two update only second.

Dry run

Why one pass is enough

Because we track both the largest and second largest at the same time, a single loop sees every value once and keeps the two we care about updated. This is O(n) — faster than sorting the whole array (O(n log n)) just to look at the second element, and it does not change the array.

Common mistakes

• Starting variables at 0. Use Integer.MIN_VALUE, or negative arrays break.
• Forgetting to slide first into second. On a new max, move the old max down.
• Counting duplicate maxes. Add x != first if you want a distinct second.
• Sorting unnecessarily. A single pass is faster and keeps the array intact.

Summary

• The second largest is the biggest value after the maximum.
• Track first and second in one pass.
• A new maximum pushes the old one into second.
• Start both at Integer.MIN_VALUE for safety.
• This is O(n) — faster than sorting and does not change the array.

Next, sort an array with bubble sort, or remove repeats in remove duplicates from an array.