496.next-greater-element-i.java

/*
 * @lc app=leetcode id=496 lang=java
 *
 * [496] Next Greater Element I
 *
 * https://leetcode.com/problems/next-greater-element-i/description/
 *
 * algorithms
 * Easy (64.58%)
 * Likes:    2031
 * Dislikes: 2529
 * Total Accepted:    188K
 * Total Submissions: 289.8K
 * Testcase Example:  '[4,1,2]\n[1,3,4,2]'
 *
 * 
 * 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:
 * 
 * All elements in nums1 and nums2 are unique.
 * The length of both nums1 and nums2 would not exceed 1000.
 * 
 * 
 */

// @lc code=start
class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        int[] result = new int[nums1.length];
		Map<Integer,Integer> map = new HashMap<Integer,Integer>();
		Stack<Integer> stack = new Stack<>();
		
		for(int i=0;i<nums2.length ; i++){
			while(!stack.isEmpty() && nums2[stack.peek()] < nums2[i]){
				map.put(nums2[stack.pop()],nums2[i]);
			}
			
			stack.push(i);
		}
		
		for(int i=0;i<nums1.length;i++){
			result[i] = map.getOrDefault(nums1[i],-1); 
		}
		
		return result;
    }
}
// @lc code=end