Gowthaman Ravi
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Stack problem | Next Greater Element I

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Gowthaman Ravi
·Feb 15, 2022·

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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.

Constraints: 1 <= nums1.length <= nums2.length <= 1000 0 <= nums1[i], nums2[i] <= 104 All integers in nums1 and nums2 are unique. All the integers of nums1 also appear in nums2.

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
                Map<Integer, Integer> map = new HashMap<>(); // map from x to next greater element of x
        Stack<Integer> stack = new Stack<>();
        for (int num : nums) {
            while (!stack.isEmpty() && stack.peek() < num)
                map.put(stack.pop(), num);
            stack.push(num);
        }   
        for (int i = 0; i < findNums.length; i++)
            findNums[i] = map.getOrDefault(findNums[i], -1);
        return findNums;
    }
}
 
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