Problem 217 (#22)

* add solution and resources

* update readme

Author: curtisbarnard

README.md

@@ -29,7 +29,7 @@ As I work through this list I figure I would make a GitHub repo with my solution
 21. [Diameter of Binary Tree #543](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/diameter-binary-tree-543.md)
 22. [Middle of the Linked List #876](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/middle-linked-list-876.md)
 23. [Maximum Depth of Binary Tree #104](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/depth-binary-tree-104.md)
-24. Contains Duplicate #217
+24. [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
 
 ### Medium
 
@@ -100,6 +100,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [Flood Fill #733](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/flood-fill-733.md)
 - [Maximum Subarray #53](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/maximum-subarray-53.md)
 - [Majority Element #169](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/majority-element-169.md)
+- [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
 
 ### Queue
 
@@ -134,6 +135,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [Climbing Stairs #70](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/climbing-stairs-70.md)
 - [Longest Palindrome #409](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/longest-palindrome-409.md)
 - [Majority Element #169](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/majority-element-169.md)
+- [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
 
 ### Binary Tree
 
@@ -154,6 +156,7 @@ Within the problems above there are several patterns that often occur. I plan to
 - [Linked List Cycle #141](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/linked-list-cycle-141.md)
 - [Reverse Linked List #206](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/reverse-linked-list-206.md)
 - [Middle of the Linked List #876](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/middle-linked-list-876.md)
+- [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
 
 ### Binary Search
 

easy/contains-duplicate-217.md

@@ -0,0 +1,77 @@
+# Contains Duplicate
+
+Page on leetcode: https://leetcode.com/problems/contains-duplicate/
+
+## Problem Statement
+
+Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.
+
+### Constraints
+
+- 1 <= nums.length <= 105
+- -109 <= nums[i] <= 109
+
+### Example
+
+```
+Input: nums = [1,2,3,1]
+Output: true
+```
+
+```
+Input: nums = [1,2,3,4]
+Output: false
+```
+
+## Solution
+
+Immediate thought is to use a hashmap and count the numbers as you iterate thru the array.
+
+### Pseudocode
+
+1. Create empty map
+2. For loop thru array
+3. If element exist in map return true
+4. else add element to map with val 1
+5. return false if it loops thru entire array
+
+### Initial Solution
+
+This solution has a time and space complexity of O(n).
+
+```javascript
+const containsDuplicate = function (nums) {
+  const map = {};
+
+  for (let i = 0; i < nums.length; i++) {
+    if (nums[i] in map) {
+      return true;
+    }
+    map[nums[i]] = 1;
+  }
+
+  return false;
+};
+```
+
+### Alternative Solution
+
+The below alternative has a time complexity O(nlogn) due to sorting, but has a space complexity of O(1). You can see explanations of different solutions here: https://www.youtube.com/watch?v=3OamzN90kPg
+
+```javascript
+const containsDuplicate = function (nums) {
+  nums.sort((a, b) => a - b);
+
+  let left = 0;
+  let right = 1;
+  while (nums[right] !== undefined) {
+    if (nums[left] === nums[right]) {
+      return true;
+    }
+    left++;
+    right++;
+  }
+
+  return false;
+};
+```