Interview Question Next Greater Element I

By Fatih KABAKCI

The problem is to figure out if there is something greater in the second array than right of the element in the first array. Once found something, it is added immediately into the result array. Otherwise -1 is added.

Problem #496 - Next Greater Element I

Description: You are given two arrays (without duplicates) nums1 and nums2 where nums1's elements are subset of nums2. Find all the next greater numbers for nums1's elements in the corresponding places of nums2.

The Next Greater Number of a number x in nums1 is the first greater number to its right in nums2. If it does not exist, output -1 for this number.

Example 1:

Input: nums1 = [4,1,2], nums2 = [1,3,4,2].
Output: [-1,3,-1]
Explanation:
a. For number 4 in the first array, you cannot find the next greater number for it in the second array, so output -1.
b. For number 1 in the first array, the next greater number for it in the second array is 3.
c. For number 2 in the first array, there is no next greater number for it in the second array, so output -1.

Example 2:
Input: nums1 = [2,4], nums2 = [1,2,3,4].
Output: [3,-1]
Explanation:
a. For number 2 in the first array, the next greater number for it in the second array is 3.
b. For number 4 in the first array, there is no next greater number for it in the second array, so output -1.

Note:
1. All elements in nums1 and nums2 are unique.
2. The length of both nums1 and nums2 would not exceed 1000.

Difficulty: Easy

Solution in Java

package com.fatihkabakci.Easy.NextGreaterElementI;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @author fkabakci 
 * Problem Description: You are given two arrays (without duplicates) nums1 and nums2 where nums1 s elements are subset of nums2. 
 * Find all the next greater numbers for nums1"s elements in the corresponding places of nums2.
 * 
 * The Next Greater Number of a number x in nums1 is the first greater number to its right in nums2. 
 * If it does not exist, output -1 for this number.
 * 
 * Example 1:
 * Input: nums1 = [4,1,2], nums2 = [1,3,4,2].
 * Output: [-1,3,-1]
 * 
 * Explanation:
 * For number 4 in the first array, you cannot find the next greater number for it in the second array, so output -1.
 * For number 1 in the first array, the next greater number for it in the second array is 3.
 * For number 2 in the first array, there is no next greater number for it in the second array, so output -1.
 * 
 * Example 2:
 * Input: nums1 = [2,4], nums2 = [1,2,3,4].
 * Output: [3,-1]
 * Explanation:
 * For number 2 in the first array, the next greater number for it in the second array is 3.
 * For number 4 in the first array, there is no next greater number for it in the second array, so output -1.
 * 
 * Note:
 * 1. All elements in nums1 and nums2 are unique.
 * 2. The length of both nums1 and nums2 would not exceed 1000.
 * 
 * Solution:
 * 1. Find the element with its index in the second array, then stop.
 * 2. Iterate the array again by starting from the index until find something lower than the element.
 * 3. Add it into the list.
 * 4. Convert list to array.
 * 5. return it.
*/
public class Solution {
	public int[] nextGreaterElement(int[] findNums, int[] nums) {
		List<Integer> list = new ArrayList<Integer>();
		for (int num : findNums) {
			int j;
			for (j = 0; j < nums.length && num != nums[j]; j++)
				;
			// stop
			int greater = -1;
			for (int k = j + 1; k < nums.length; k++)
				if (num < nums[k]) {
					greater = nums[k];
					break;
				}
			list.add(greater);
		}
		
		// copy list to array
		int[] resultSet = new int[list.size()];
		for (int i = 0; i < list.size(); i++)
			resultSet[i] = list.get(i);
		return resultSet;
	}

	public static void main(String[] args) {
		Solution s = new Solution();	
		System.out.println(Arrays.toString(s.nextGreaterElement(new int[] {4, 1, 2}, new int[] {1, 3, 4, 2})));
	}
}

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