3871. Count Commas in Range II

Medium

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 <= 10^15

解答

Solution

/**
 * @param {number} n
 * @return {number}
 */
var countCommas = function (n) {
  let base = 1000;
  let result = 0;

  while (n >= base) {
    result += n - base + 1;
    base *= 1000;
  }

  return result;
};

解題思路

要算出總共有幾個逗號。
但測資比較大，所以不能和上一題目一樣暴力解。
這次改用計算ㄉ。

var countCommas = function (n) {
  let base = 1000; // 因為每多三個 0 才會多一個逗號，故 base 設定為 1000
  let result = 0; // 總共有幾個逗號，最少是 0 個

  while (n >= base) {
    result += n - base + 1; // 算出這輪共有幾個逗號
    base *= 1000; // 進到下一輪，base 需要乘上 1000
  }

  return result;
};

心得

比想像中還容易理解一點。