1608. Special Array With X Elements Greater Than or Equal X

Description

You are given an array nums of non-negative integers. nums is considered special if there exists a number x such that there are exactly x numbers in nums that are greater than or equal to x.

Notice that x does not have to be an element in nums.

Return x if the array is special, otherwise, return -1. It can be proven that if nums is special, the value for x is unique.

 

Example 1:

Input: nums = [3,5]
Output: 2
Explanation: There are 2 values (3 and 5) that are greater than or equal to 2.

Example 2:

Input: nums = [0,0]
Output: -1
Explanation: No numbers fit the criteria for x.
If x = 0, there should be 0 numbers >= x, but there are 2.
If x = 1, there should be 1 number >= x, but there are 0.
If x = 2, there should be 2 numbers >= x, but there are 0.
x cannot be greater since there are only 2 numbers in nums.

Example 3:

Input: nums = [0,4,3,0,4]
Output: 3
Explanation: There are 3 values that are greater than or equal to 3.

 

Constraints:

• 1 <= nums.length <= 100
• 0 <= nums[i] <= 1000

 

Solutions

Solution: Binary Search

• Time complexity: O(nlogn)
• Space complexity: O(1)

 

JavaScript

/**
 * @param {number[]} nums
 * @return {number}
 */
const specialArray = function (nums) {
  const n = nums.length;
  const findGreaterCount = x => {
    let left = 0;
    let right = n - 1;

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

      nums[mid] >= x ? (left = mid + 1) : (right = mid - 1);
    }
    return left;
  };

  nums.sort((a, b) => b - a);

  for (let x = 1; x <= n; x++) {
    if (x === findGreaterCount(x)) return x;
  }
  return -1;
};