[yoonthecoder] Week 1

contains-duplicate/yoonthecoder.js

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+var containsDuplicate = function (nums) {
+  // Create a set from the nums array. Since Sets only allow unique values, any duplicates will be removed.
+  const set = new Set(nums);
+  // Compare the size of the set and the length of the original array.- if the size of the set is smaller than the length of the original array('nums'), it means there were duplicates.
+
+  return set.size < nums.length;
+};
+
+// Time Complexity: O(n); - adding elements to the Set & compare sizes
+// Space Complexity: O(n)

longest-consecutive-sequence/yoonthecoder.js

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+var longestConsecutive = function (nums) {
+  // remove the duplicates from the array and sort it in an ascending order.
+  const setArray = [...new Set(nums)];
+  const sortedArray = setArray.sort((a, b) => a - b);
+  // create a set to store streak lengths, even when count resets.
+  const countSet = new Set();
+  let count = 0;
+  for (let i = 0; i < sortedArray.length; i++) {
+    if (sortedArray[i] + 1 == sortedArray[i + 1]) {
+      count += 1;
+      countSet.add(count);
+    } else {
+      count = 0;
+    }
+  }
+
+  return nums.length === 0 ? 0 : countSet.size + 1;
+};
+
+// Time complexity: O(nlogn) => TODO: need to improve this to O(n)
+// Space complexity: O(n)

top-k-frequent-elements/yoonthecoder.js

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+var topKFrequent = function (nums, k) {
+  // 1. count the frequency of each number in the array
+  const map = new Map();
+
+  // iterating through the array to count how many times each num appears.
+  for (const num of nums) {
+    // if the num already exists in the map, increment its count
+    if (map.has(num)) {
+      map.set(num, map.get(num) + 1);
+    } // otherwise, set it to 1
+    else map.set(num, 1);
+  }
+
+  // 2.create an array to store the freqeuncy numbers
+  const freqArr = [];
+  for (const [num, freq] of map) {
+    freqArr.push([num, freq]);
+  }
+  // sort in descending order by frequency
+  freqArr.sort((a, b) => b[1] - a[1]);
+  return freqArr.slice(0, k).map(([num]) => num);
+};
+
+// Time complexity: O(nlogn)
+// Space complexity: O(n)

valid-palindrome/yoonthecoder.js

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+var isPalindrome = function (s) {
+  // remove any special characters and space from the string
+  const formattedString = s.toLowerCase().replace(/[^a-zA-Z0-9]/g, '');
+  // use split() method to separate each charaters and put them in an array - reverse it - concatenate
+  const reversedString = formattedString.split('').reverse().join('');
+
+  return reversedString === formattedString;
+};
+
+// time complexity: O(n)
+// space complexity: O(n)