觀前提醒：

1. 我預設大家已經先思考並分析過題目，沒啥想法才開始 google 找解題靈感。若無，建議每題先花 1~2 顆番茄鐘的時間來分析題目比較好。可參考番茄鐘工作法。
2. 承上，既然已經有思考過了，那我這邊直接 po 題目 + 解題想法 + code +心得 。若已經在 code 內有足夠的註解了，那我可能解題想法 & 心得的部分就不會寫太多，免得干擾你的思考。
3. 所有解法都是已經取得系統的 Accepted，但或許不是最優解法，請多包涵。
4. 若對於解法不太懂，可以嘗試用 Chrome 的 debugger 來試跑看看 (教學文)
5. 最後，歡迎在下面留言指教~教學相長才會進步歐~

題目

Write a function that takes an unsigned integer and return the number of '1' bits it has (also known as the Hamming weight).

Example 1:

Input: 00000000000000000000000000001011
Output: 3
Explanation: The input binary string 00000000000000000000000000001011 has a total of three '1' bits.

Example 2:

Input: 00000000000000000000000010000000
Output: 1
Explanation: The input binary string 00000000000000000000000010000000 has a total of one '1' bit.

Example 3:

Input: 11111111111111111111111111111101
Output: 31
Explanation: The input binary string 11111111111111111111111111111101 has a total of thirty one '1' bits.

Note:

Note that in some languages such as Java, there is no unsigned integer type. In this case, the input will be given as signed integer type and should not affect your implementation, as the internal binary representation of the integer is the same whether it is signed or unsigned.
In Java, the compiler represents the signed integers using 2's complement notation. Therefore, in Example 3 above the input represents the signed integer -3.

Follow up:

If this function is called many times, how would you optimize it?

解題想法

• 將數字n轉為2進位字串，並分拆存成陣列。(Number.prototype.toString())
• 跑迴圈找出有多少 1 。

CODE

/**
 * @param {number} n - a positive integer
 * @return {number}
 */
var hammingWeight = function (n) {
  let countOf1 = 0;
  let arrOfN = n.toString(2).split("");
  for (i = 0; i < arrOfN.length; i++) {
    if (arrOfN[i] === "1") {
      countOf1++;
    }
  }
  return countOf1;
};

心得

看到這題的標籤是 easy，在參考其他語言的解法後，我認為應該是 JS 本身 Number本身的特性。也就是超過顯示上限後，會自動轉成科學記號所致。

所以這邊為求保險，我還是先用內建函式來處理，以確保輸出結果是符合題目要求這樣。

謝謝大家的收看，LeetCode 小學堂我們下次見~