3870. Count Commas in Range

Easy

題目描述

Description

You are given an integer n.

Return the total number of commas used when writing all integers from [1, n] (inclusive) in standard number formatting.

In standard formatting:

• A comma is inserted after every three digits from the right.
• Numbers with fewer than 4 digits contain no commas.

Example 1:

Input: n = 1002

Output: 3

Explanation:

The numbers "1,000", "1,001", and "1,002" each contain one comma, giving a total of 3.

Example 2:

Input: n = 998

Output: 0

Explanation:

All numbers from 1 to 998 have fewer than four digits. Therefore, no commas are used.

Constraints:

• 1 <= n <= 10^5

解答

Solution

/**
 * @param {number} n
 * @return {number}
 */
var countCommas = function (n) {
  if (n === 1000000) return 999002;
  if (n < 1000) return 0;
  return n - 1000 + 1;
};

解題思路

要算出總共有幾個逗號。
然後測資很少，只有 1 <= n <= $10^5$，所以要考慮的只有三種狀況。

var countCommas = function (n) {
  if (n === 1000000) return 999002; // 最大值 1000000 的狀況，會變成 1,000,000，有兩個逗號，可以先用 n - 1000 + 1 算出除他之外其他值總和，最後再加二，會變成 999002。
  if (n < 1000) return 0; // 小於 1000 的狀況，沒有逗號
  return n - 1000 + 1; // 介於 1000 到最大值的狀況，可以用 n - 1000 + 1 這個公式算出。
};

心得

甘單，不到五分鐘搞定😎