Question

Formatted question description: https://leetcode.ca/all/1636.html

 1636. 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

Algorithm

Use map for frequency count.

Code

Java

public class Sort_Array_by_Increasing_Frequency {

    class Solution {

        public int[] frequencySort(int[] nums) {
            Map<Integer, Integer> map = new HashMap<>();
            // count frequency of each number
            Arrays.stream(nums).forEach(n -> map.put(n, map.getOrDefault(n, 0) + 1));
            // custom sort
            return Arrays.stream(nums).boxed()
                .sorted((a,b) -> map.get(a) != map.get(b) ? map.get(a) - map.get(b) : b - a)
                .mapToInt(n -> n)
                .toArray();
        }
    }
}

Java

class Solution {
    public int[] frequencySort(int[] nums) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int num : nums) {
            int count = map.getOrDefault(num, 0) + 1;
            map.put(num, count);
        }
        List<Integer> list = new ArrayList<Integer>();
        for (int num : nums)
            list.add(num);
        Collections.sort(list, new Comparator<Integer>() {
            public int compare(Integer num1, Integer num2) {
                int count1 = map.get(num1), count2 = map.get(num2);
                if (count1 != count2)
                    return count1 - count2;
                else
                    return num2 - num1;
            }
        });
        int length = nums.length;
        int[] sorted = new int[length];
        for (int i = 0; i < length; i++)
            sorted[i] = list.get(i);
        return sorted;
    }
}

All Problems

All Solutions