LC 496 Next Greater Element I

Implementation

// 496. Next Greater Element I
// 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.

// Follow up: Could you find an O(nums1.length + nums2.length) solution?
var nextGreaterElement = (nums1, nums2) => {
   // simpler approach
   let stack = [];
   let map = new Map();

   for(let num of nums2){
      while(stack.length && num > stack[stack.length-1]){
            map.set(stack.pop(),num);
      }
      stack.push(num);
   }
   while(stack.length){
      map.set(stack.pop(),-1);
   }
   return nums1.map(item => map.get(item));
}
console.log(nextGreaterElement([4,1,2],[1,3,4,2]));
console.log(nextGreaterElement([2,4],[1,2,3,4]));
console.log(nextGreaterElement([1,3,5,2,4],[6,5,4,3,2,1,7]));

Tests

// Test stub for 01.496.NextGreaterElement

describe('01.496.NextGreaterElement', () => {
  it('should have a test stub', () => {
    expect(true).toBe(true);
  });
});