496. Next Greater Element I

You are given two integer arrays nums1 and nums2 both of unique elements, where nums1 is a 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, return -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.

 

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?

Rust Solution

struct Solution;

use std::collections::HashMap;

impl Solution {
    fn next_greater_element(nums1: Vec<i32>, nums2: Vec<i32>) -> Vec<i32> {
        let mut stack: Vec<i32> = vec![];
        let mut hm: HashMap<i32, i32> = HashMap::new();
        for x in nums2 {
            while let Some(last) = stack.pop() {
                if last > x {
                    stack.push(last);
                    break;
                } else {
                    hm.insert(last, x);
                }
            }
            stack.push(x);
        }
        nums1.iter().map(|x| *hm.get(x).unwrap_or(&-1)).collect()
    }
}

#[test]
fn test() {
    let nums1 = vec![4, 1, 2];
    let nums2 = vec![1, 3, 4, 2];
    let res = vec![-1, 3, -1];
    assert_eq!(Solution::next_greater_element(nums1, nums2), res);
    let nums1 = vec![2, 4];
    let nums2 = vec![1, 2, 3, 4];
    let res = vec![3, -1];
    assert_eq!(Solution::next_greater_element(nums1, nums2), res);
}

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