notes/OJ notes/pages/Leetcode Next-Greater-Element-I.md

# Leetcode Next-Greater-Element-I

#### 2022-07-27 10:55

> ##### Data structures:
>
> #DS #stack #hash_table
>
> ##### Difficulty:
>
> #coding_problem #difficulty_easy
>
> ##### Additional tags:
>
> #leetcode
>
> ##### Revisions:
>
> N/A

##### Related topics:

##### Links:

- [Link to problem](https://leetcode.com/problems/next-greater-element-i/)

---

### Problem

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._

#### Examples

**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

### Thoughts

> [!summary]
> This is a problem using the traits of #stack.

This problem can be divided into two sub-problems:

- given a subset number, return its location in the parent set -> hash table
- given a array, return the next greater element in the array after it -> stack

#### How is the stack used?

Stack is FILO, which means, when iterating from the last element, if you push a number greater into it, it will be used last, ideal for finding the **next** biggest one.

### Solution

```cpp
class Solution {
public:
  vector<int> nextGreaterElement(vector<int> &nums1, vector<int> &nums2) {
    // Hash table to link the subsets.
    unordered_map<int, int> greater;
    stack<int> st;

    for (int i = nums2.size() - 1; i >= 0; i--) {
      while (!st.empty() && st.top() < nums2[i]) {
        st.pop();
      }
      if (!st.empty()) {
        greater[nums2[i]] = st.top();
      }

      st.push(nums2[i]);
    }

    // double pointers can't be used, because nums doesn't have a order.
    vector<int> ans(nums1.size());
    for (int i = 0, size1 = nums1.size(); i < size1; i++) {
      if (greater.find(nums1[i]) != greater.end()) {
        ans[i] = greater[nums1[i]];
      } else {
        ans[i] = -1;
      }
    }

    return ans;
  }
};
```