Description (leetcode)

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] <= 10^4
  • 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?

submission

impl Solution {
    pub fn next_greater_element(nums1: Vec<i32>, nums2: Vec<i32>) -> Vec<i32> {
        let mut ans = vec![-1; nums1.len()];
        let indices = nums1
            .iter()
            .copied()
            .enumerate()
            .map(|(k, v)| (v, k))
            .collect::<std::collections::HashMap<i32, usize>>();
        let mut stack = Vec::with_capacity(nums2.len());
        for val in nums2.into_iter().rev() {
            while stack.pop_if(|&mut _v| _v <= val).is_some() {}
            if let Some(&_v) = stack.last()
                && let Some(&idx) = indices.get(&val)
            {
                ans[idx] = _v;
            }
            stack.push(val);
        }
        ans
    }
}