Next Greater Element in Java: Explanation & Practice

Problem summary

For each element in the array, find the next greater element to its right. Use monotonic decreasing stack.

Starter code

public class Main {
    public static void main(String[] args) {
        int[] nums = {4, 5, 2, 25};
        
        // Monotonic stack: keep decreasing elements
        // When we find a greater element, pop and assign
        // Print: 5 25 25 -1
    }
}

Expected output and test cases

• [4,5,2,25]

  5 25 25 -1

• [3,2,1] → all -1

  -1 -1 -1

• [1,2,3]

  2 3 -1

Hints

1. Use stack to store indices of elements waiting for their next greater
2. For each new element, pop all smaller elements from stack
3. Those popped elements found their next greater (current element)
4. Push current element's index onto stack

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