1870. Minimum Speed to Arrive on Time

Description

You are given a floating-point number hour, representing the amount of time you have to reach the office. To commute to the office, you must take n trains in sequential order. You are also given an integer array dist of length n, where dist[i] describes the distance (in kilometers) of the ith train ride.

Each train can only depart at an integer hour, so you may need to wait in between each train ride.

• For example, if the 1st train ride takes 1.5 hours, you must wait for an additional 0.5 hours before you can depart on the 2nd train ride at the 2 hour mark.

Return the minimum positive integer speed (in kilometers per hour) that all the trains must travel at for you to reach the office on time, or -1 if it is impossible to be on time.

Tests are generated such that the answer will not exceed 107 and hour will have at most two digits after the decimal point.

 

Example 1:

Input: dist = [1,3,2], hour = 6
Output: 1
Explanation: At speed 1:
- The first train ride takes 1/1 = 1 hour.
- Since we are already at an integer hour, we depart immediately at the 1 hour mark. The second train takes 3/1 = 3 hours.
- Since we are already at an integer hour, we depart immediately at the 4 hour mark. The third train takes 2/1 = 2 hours.
- You will arrive at exactly the 6 hour mark.

Example 2:

Input: dist = [1,3,2], hour = 2.7
Output: 3
Explanation: At speed 3:
- The first train ride takes 1/3 = 0.33333 hours.
- Since we are not at an integer hour, we wait until the 1 hour mark to depart. The second train ride takes 3/3 = 1 hour.
- Since we are already at an integer hour, we depart immediately at the 2 hour mark. The third train takes 2/3 = 0.66667 hours.
- You will arrive at the 2.66667 hour mark.

Example 3:

Input: dist = [1,3,2], hour = 1.9
Output: -1
Explanation: It is impossible because the earliest the third train can depart is at the 2 hour mark.

 

Constraints:

• n == dist.length
• 1 <= n <= 105
• 1 <= dist[i] <= 105
• 1 <= hour <= 109
• There will be at most two digits after the decimal point in hour.

 

Solutions

Solution: Binary Search

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

 

JavaScript

/**
 * @param {number[]} dist
 * @param {number} hour
 * @return {number}
 */
const minSpeedOnTime = function (dist, hour) {
  const size = dist.length;

  if (size - 1 >= hour) return -1;
  const MAX_SPEED = 10 ** 7;
  const calculateReachTime = speed => {
    let result = 0;

    for (let index = 0; index < size - 1; index++) {
      result += Math.ceil(dist[index] / speed);
    }
    return result + dist[size - 1] / speed;
  };
  let left = 1;
  let right = MAX_SPEED;

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

    reachTime > hour ? (left = mid + 1) : (right = mid);
  }
  return left;
};