19_NGE1.java

// The next greater element of some element x in an array is the first greater element that is to the right of x in the same array.

// You are given two distinct 0-indexed integer arrays nums1 and nums2, where nums1 is a subset of nums2.

// For each 0 <= i < nums1.length, find the index j such that nums1[i] == nums2[j] and determine the next greater element of nums2[j] in nums2. If there is no next greater element, then the answer for this query is -1.

// Return an array ans of length nums1.length such that ans[i] is the next greater element as described above.

 

// Example 1:

// Input: nums1 = [4,1,2], nums2 = [1,3,4,2]
// Output: [-1,3,-1]
// Explanation: The next greater element for each value of nums1 is as follows:
// - 4 is underlined in nums2 = [1,3,4,2]. There is no next greater element, so the answer is -1.
// - 1 is underlined in nums2 = [1,3,4,2]. The next greater element is 3.
// - 2 is underlined in nums2 = [1,3,4,2]. There is no next greater element, so the answer is -1.
// Example 2:

// Input: nums1 = [2,4], nums2 = [1,2,3,4]
// Output: [3,-1]
// Explanation: The next greater element for each value of nums1 is as follows:
// - 2 is underlined in nums2 = [1,2,3,4]. The next greater element is 3.
// - 4 is underlined in nums2 = [1,2,3,4]. There is no next greater element, so the answer is -1.
 









// Optimal solution 

import java.util.*;

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        Stack<Integer> stack = new Stack<>();
        Map<Integer, Integer> map = new HashMap<>();
        
        for (int n : nums2) {
            while (!stack.isEmpty() && stack.peek() < n) {
                map.put(stack.pop(), n);
            }
            stack.push(n);
        }
        
        while (!stack.isEmpty()) {
            map.put(stack.pop(), -1);
        }
        
        int[] res = new int[nums1.length];
        for (int i = 0; i < nums1.length; i++) {
            res[i] = map.get(nums1[i]);
        }
        return res;
    }
}









// Optimal solution

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        int n = nums2.length;
        int[] stack = new int[n];
        int top = -1;
        int[] next = new int[n];

        for (int i = 0; i < n; i++) {
            next[i] = -1;
        }

        for (int i = 0; i < n; i++) {
            while (top != -1 && nums2[stack[top]] < nums2[i]) {
                next[stack[top--]] = nums2[i];
            }
            stack[++top] = i;
        }

        int[] result = new int[nums1.length];
        for (int i = 0; i < nums1.length; i++) {
            for (int j = 0; j < n; j++) {
                if (nums2[j] == nums1[i]) {
                    result[i] = next[j];
                    break;
                }
            }
        }
        return result;
    }
}