2080. Range Frequency Queries

Description

Design a data structure to find the frequency of a given value in a given subarray.

The frequency of a value in a subarray is the number of occurrences of that value in the subarray.

Implement the RangeFreqQuery class:

• RangeFreqQuery(int[] arr) Constructs an instance of the class with the given 0-indexed integer array arr.
• int query(int left, int right, int value) Returns the frequency of value in the subarray arr[left...right].

A subarray is a contiguous sequence of elements within an array. arr[left...right] denotes the subarray that contains the elements of nums between indices left and right (inclusive).

 

Example 1:

Input
["RangeFreqQuery", "query", "query"]
[[[12, 33, 4, 56, 22, 2, 34, 33, 22, 12, 34, 56]], [1, 2, 4], [0, 11, 33]]
Output
[null, 1, 2]

Explanation
RangeFreqQuery rangeFreqQuery = new RangeFreqQuery([12, 33, 4, 56, 22, 2, 34, 33, 22, 12, 34, 56]);
rangeFreqQuery.query(1, 2, 4); // return 1. The value 4 occurs 1 time in the subarray [33, 4]
rangeFreqQuery.query(0, 11, 33); // return 2. The value 33 occurs 2 times in the whole array.

 

Constraints:

• 1 <= arr.length <= 105
• 1 <= arr[i], value <= 104
• 0 <= left <= right < arr.length
• At most 105 calls will be made to query

 

Solutions

Solution: Hash Table + Binary Search

• Time complexity: O(logn)
• Space complexity: O(n)

 

JavaScript

/**
 * @param {number[]} arr
 */
const RangeFreqQuery = function (arr) {
  this.frequencyMap = arr.reduce((map, value, index) => {
    const indices = map.get(value);

    indices ? indices.push(index) : map.set(value, [index]);
    return map;
  }, new Map());
};

function binarySearch(arr, target, lower = false) {
  let left = 0;
  let right = arr.length;
  const isBigger = value => (lower ? target > value : target >= value);

  while (left < right) {
    const middle = Math.floor((left + right) / 2);

    isBigger(arr[middle]) ? (left = middle + 1) : (right = middle);
  }
  return left;
}

/**
 * @param {number} left
 * @param {number} right
 * @param {number} value
 * @return {number}
 */
RangeFreqQuery.prototype.query = function (left, right, value) {
  if (!this.frequencyMap.has(value)) return 0;
  const indices = this.frequencyMap.get(value);

  return binarySearch(indices, right) - binarySearch(indices, left, true);
};

/**
 * Your RangeFreqQuery object will be instantiated and called as such:
 * var obj = new RangeFreqQuery(arr)
 * var param_1 = obj.query(left,right,value)
 */