init(Easy/Array): add solution for 1636. Sort Array by Increasing Frequency

init(Medium/Hash Table): add solution for 451. Sort Characters By Frequency

Easy/Array/1636.sort-array-by-increasing-frequency.js

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+/*
+ * 1636. Sort Array by Increasing Frequency
+ * Difficulty: Easy
+ * https://leetcode.com/problems/sort-array-by-increasing-frequency/
+ *
+ * ──────────────────────────────────────────────────
+ *
+ * Given an array of integers nums, sort the array in increasing order
+ * based on the frequency of the values. If multiple values have the same
+ * frequency, sort them in decreasing order.
+ *
+ * Return the sorted array.
+ *
+ *
+ *
+ * Example 1:
+ *
+ * Input: nums = [1,1,2,2,2,3]
+ * Output: [3,1,1,2,2,2]
+ * Explanation: '3' has a frequency of 1, '1' has a frequency of 2, and
+ * '2' has a frequency of 3.
+ *
+ * Example 2:
+ *
+ * Input: nums = [2,3,1,3,2]
+ * Output: [1,3,3,2,2]
+ * Explanation: '2' and '3' both have a frequency of 2, so they are
+ * sorted in decreasing order.
+ *
+ * Example 3:
+ *
+ * Input: nums = [-1,1,-6,4,5,-6,1,4,1]
+ * Output: [5,-1,4,4,-6,-6,1,1,1]
+ *
+ *
+ *
+ * Constraints:
+ *
+ * 	• 1 <= nums.length <= 100
+ *
+ * 	• -100 <= nums[i] <= 100
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number[]}
+ */
+var frequencySort = function (nums) {
+  const freq = {};
+  for (let n of nums) freq[n] = (freq[n] || 0) + 1;
+  return nums.sort((a, b) => freq[a] - freq[b] || b - a);
+};

Medium/Hash Table/451.sort-characters-by-frequency.js

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+/*
+ * 451. Sort Characters By Frequency
+ * Difficulty: Medium
+ * https://leetcode.com/problems/sort-characters-by-frequency/
+ *
+ * ──────────────────────────────────────────────────
+ *
+ * Given a string s, sort it in decreasing order based on the frequency
+ * of the characters. The frequency of a character is the number of times
+ * it appears in the string.
+ *
+ * Return the sorted string. If there are multiple answers, return any
+ * of them.
+ *
+ *
+ *
+ * Example 1:
+ *
+ * Input: s = "tree"
+ * Output: "eert"
+ * Explanation: 'e' appears twice while 'r' and 't' both appear once.
+ * So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also
+ * a valid answer.
+ *
+ * Example 2:
+ *
+ * Input: s = "cccaaa"
+ * Output: "aaaccc"
+ * Explanation: Both 'c' and 'a' appear three times, so both "cccaaa"
+ * and "aaaccc" are valid answers.
+ * Note that "cacaca" is incorrect, as the same characters must be
+ * together.
+ *
+ * Example 3:
+ *
+ * Input: s = "Aabb"
+ * Output: "bbAa"
+ * Explanation: "bbaA" is also a valid answer, but "Aabb" is incorrect.
+ * Note that 'A' and 'a' are treated as two different characters.
+ *
+ *
+ *
+ * Constraints:
+ *
+ * 	• 1 <= s.length <= 5 * 10^5
+ *
+ * 	• s consists of uppercase and lowercase English letters and digits.
+ */
+
+/**
+ * @param {string} s
+ * @return {string}
+ */
+var frequencySort = function (s) {
+  // count the frequency of each character
+  const freq = {};
+  for (let c of s) freq[c] = (freq[c] || 0) + 1;
+
+  // sort the characters by frequency
+  return s
+    .split("")
+    .sort((a, b) => freq[b] - freq[a] || a.localeCompare(b))
+    .join("");
+};