Sandbox::https://github.com/prashanthigupta/leetcode-algorithms/blob/master/src/NextGreaterElementI.java

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import java.util.Arrays;

import java.util.HashMap;

import java.util.Map;

import java.util.Stack;

public class NextGreaterElementI {

public static int[] nextGreaterElement(int[] nums1, int[] nums2) {

Map<Integer, Integer> nextGreatestElement = getNextGreaterElementsFor(nums2);

int[] result = new int[nums1.length];

for (int index = 0 ; index < nums1.length ; index++) {

result[index] = nextGreatestElement.get(nums1[index]);

}

return result;

}

private static Map<Integer, Integer> getNextGreaterElementsFor(int[] array) {

Map<Integer, Integer> result = new HashMap<>();

Stack<Integer> stack = new Stack<>();

for (int element : array) {

while (!stack.isEmpty() && element > stack.peek()) {

result.put(stack.peek(), element);

stack.pop();

}

stack.push(element);

}

while (!stack.isEmpty()) {

result.put(stack.pop(), -1);

}

return result;

}

public static void main(String[] args) {

int[] result = nextGreaterElement(new int[] {4, 1, 2}, new int[] {1, 3, 4, 2});

System.out.println(Arrays.toString(result));

}