# 14 Days Study Plan to Crack Algo I

## 2022.03.20

# Day 13

# 231. Power of Two

Given an integer `n`

, return *true** if it is a power of two. Otherwise, return *

.*false*

An integer `n`

is a power of two, if there exists an integer `x`

such that `n == 2x`

.

**Example 1:**

**Input:** n = 1

**Output:** true

**Explanation: **20 = 1

**Example 2:**

**Input:** n = 16

**Output:** true

**Explanation: **24 = 16

**Example 3:**

**Input:** n = 3

**Output:** false

**Constraints:**

`-231 <= n <= 231 - 1`

**Follow up:** Could you solve it without loops/recursion?

# 191. Number of 1 Bits

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

**Note:**

- Note that in some languages, such as Java, there is no unsigned integer type. In this case, the input will be given as a signed integer type. It should not affect your implementation, as the integer’s internal binary representation 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**, the input represents the signed integer.`-3`

.

**Example 1:**

**Input:** n = 00000000000000000000000000001011

**Output:** 3

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

**Example 2:**

**Input:** n = 00000000000000000000000010000000

**Output:** 1

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

**Example 3:**

**Input:** n = 11111111111111111111111111111101

**Output:** 31

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

**Constraints:**

- The input must be a
**binary string**of length`32`

.

**Follow up:** If this function is called many times, how would you optimize it?

Bit solution: https://leetcode.com/problems/number-of-1-bits/discuss/921847/Three-JS-solutions