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

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+# 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:
+```expander
+tag:#stack
+```
+ 
+ 
+
+##### 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