# Problem

Given a 32-bit signed integer, reverse digits of an integer.

Example 1:

Input: 123
Output: 321


Example 2:

Input: -123
Output: -321


Example 3:

Input: 120
Output: 21


Note:
Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231,  231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

# Solution: Simulation

Reverse digit by digit. Be careful about the overflow and negative numbers (especially in Python)

Time complexity: O(log(x)) ~ O(1)

Space complexity: O(log(x)) ~ O(1)

# Problem

Given an array, rotate the array to the right by k steps, where k is non-negative.

Example 1:

Input: [1,2,3,4,5,6,7] and k = 3
Output: [5,6,7,1,2,3,4]
Explanation: rotate 1 steps to the right: [7,1,2,3,4,5,6] rotate 2 steps to the right: [6,7,1,2,3,4,5]
rotate 3 steps to the right: [5,6,7,1,2,3,4]

Example 2:

Input:[-1,-100,3,99] and k = 2
Output: [3,99,-1,-100]
Explanation: rotate 1 steps to the right: [99,-1,-100,3] rotate 2 steps to the right: [3,99,-1,-100]

Note:

• Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.
• Could you do it in-place with O(1) extra space?

# Solution 1: Simulate rotation with three reverses.

If k >= n, rotating k times has the same effect as rotating k % n times.

[1,2,3,4,5,6,7], K = 3

[5,6,7,1,2,3,4]

We can simulate the rotation with three reverses.

1. reverse the whole array O(n) [7,6,5,4,3,2,1]
2. reverse the left part 0 ~ k – 1 O(k) [5,6,7,4,3,2,1]
3. reverse the right part k ~ n – 1 O(n-k) [5,6,7,1,2,3,4]

Time complexity: O(n)

Space complexity: O(1) in-place

C++

# Problem

Write a function that takes a string as input and reverse only the vowels of a string.

Example 1:
Given s = “hello”, return “holle”.

Example 2:
Given s = “leetcode”, return “leotcede”.

Note:
The vowels does not include the letter “y”.

# Problem

https://leetcode.com/problems/reverse-string-ii/description/

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.Example:

Input: s = "abcdefg", k = 2
Output: "bacdfeg"


Restrictions:

1. The string consists of lower English letters only.
2. Length of the given string and k will in the range [1, 10000]

C++

# Related Problems

Problem:

Solution 1:

Tracking prev / curr / next node

Time complexity: O(n)

Space complexity: O(1)

C++

Java

Python 3