LeetCode 2454: Next Greater Element IV Solution

Master LeetCode problem 2454 (Next Greater Element IV), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

2454. Next Greater Element IV

Hard

Array Binary Search Stack Sorting Heap (Priority Queue)Monotonic Stack

Problem Explanation

Explanation

We can solve this problem using a stack to keep track of elements in decreasing order.
We iterate through the array from right to left and maintain a stack of elements.
For each element, we pop elements from the stack as long as they are less than the current element. The top of the stack will be the second greater element for the current element.
If the stack becomes empty before finding the second greater element, we set it to -1.
We store the result in an array and return it.

Time Complexity: O(n) Space Complexity: O(n)

Solution Code

import java.util.Stack;

class Solution {
    public int[] nextGreaterElement(int[] nums) {
        int n = nums.length;
        int[] result = new int[n];
        Stack<Integer> stack = new Stack<>();

        for (int i = n - 1; i >= 0; i--) {
            while (!stack.isEmpty() && nums[i] >= stack.peek()) {
                stack.pop();
            }

            result[i] = stack.isEmpty() ? -1 : stack.peek();
            stack.push(nums[i]);
        }

        return result;
    }
}

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