[선재] Week1 문제 풀이

contains-duplicate/sunjae95.js

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+/**
+ * @description
+ * time complexity: O(n)
+ * space complexity: O(n)
+ * approach/strategy:
+ * 1. brute force + hash table
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {boolean}
+ */
+var containsDuplicate = function (nums) {
+  const hashTable = new Set();
+
+  for (const num of nums) {
+    if (hashTable.has(num)) return true;
+    hashTable.add(num);
+  }
+
+  return false;
+};

kth-smallest-element-in-a-bst/sunjae95.js

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+/**
+ * @description
+ * time complexity: O(N)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. BFS, DFS
+ * 2. Brute force
+ *
+ * strategy:
+ * inOrder search
+ *
+ * reason:
+ * tree features
+ */
+var kthSmallest = function (root, k) {
+  let answer = 0;
+
+  inOrder(root, (value) => {
+    k -= 1;
+    if (k > 0) return false;
+    if (k === 0) answer = value;
+    return true;
+  });
+
+  return answer;
+};
+
+function inOrder(tree, isEnd) {
+  if (tree.left) inOrder(tree.left, isEnd);
+  if (isEnd(tree.val)) return;
+  if (tree.right) inOrder(tree.right, isEnd);
+}

number-of-1-bits/sunjae95.js

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+/**
+ * @description
+ * time complexity: O(logN)
+ * space complexity: O(1)
+ * approach/strategy:
+ * 1. decimal to binary
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var hammingWeight = function (n) {
+  let answer = 0;
+
+  while (n > 0) {
+    answer += n % 2;
+    n = Math.floor(n / 2);
+  }
+
+  return answer;
+};

top-k-frequent-elements/sunjae95.js

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+/**
+ * @description
+ * time complexity: O(N logN)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. javascript sort method
+ * 2. priority queue
+ *
+ * strategy:
+ * javascript sort method
+ *
+ * reason:
+ * javascript sort method is easier to implement.
+ */
+
+var topKFrequent = function (nums, k) {
+  const answer = [];
+  const array = [];
+  const hashTable = new Map();
+
+  nums.forEach((num) => hashTable.set(num, (hashTable.get(num) ?? 0) + 1));
+
+  hashTable.forEach((count, number) => array.push({ number, count }));
+
+  array.sort((a, b) => b.count - a.count);
+
+  for (let i = 0; i < k; i++) answer.push(array[i].number);
+
+  return answer;
+};