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LeetCode 151 - Reverse Words in a String

Programming Languages:

  • Python3
  • SQL Server Transact-SQL

Difficulty: ⭐⭐⭐

Problem Description

Requirement

Given an input string s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Examples

Example 1

Input: s = "the sky is blue"

Output: "blue is sky the"

Example 2

Input: s = " hello world "

Output: "world hello"

Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3

Input: s = "a good example"

Output: "example good a"

Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Constraints

  • 1 <= s.length <= 104
  • s contains English letters (upper-case and lower-case), digits, and spaces ' '.
  • There is at least one word in s.

Solutions

Python Solution

Reversing words in a string can be accomplished using three steps:

  1. Split the string using a space as the separator and store in a list.
  2. Reverse the order of the list.
  3. Merge the list of strings separating each string with a space.
class Solution:
    def reverseWords(self, s: str) -> str:
        # Step 1: Split the string into a list of words
        words = s.split()

        # Step 2: Reverse the order of the list of words
        reversed_words = words[::-1]

        # Step 3: Return the joined reversed list of words separated by spaces
        return ' '.join(reversed_words)
  • Runtime: 31ms
  • Beats: 87.17% as of August 15, 2024
  • Memory: 16.55MB
  • Beats: 87.10% as of August 15, 2024

SQL Server Solution

To reverse words in a string using SQL Server, the same steps will be performed:

  1. Split the string using a space as the separator and store in a list.
  2. Reverse the order of the list.
  3. Merge the list of strings separating each string with a space.

In SQL Server, to split a string that is separated by space, the STRING_SPLIT string function can be used:

The STRING_SPLIT string function is a table-valued function that splits a string into rows of substrings, based on a specified separator character.

Syntax

STRING_SPLIT ( string , separator [ , enable_ordinal ] )

Arguments

string

An expression of any character type (for example, nvarchar, varchar, nchar, or char).

separator

A single character expression of any character type (for example, nvarchar(1), varchar(1), nchar(1), or char(1)) that is used as separator for concatenated substrings.

enable_ordinal

An int or bit expression that serves as a flag to enable or disable the ordinal output column. A value of 1 enables the ordinal column. If enable_ordinal is omitted, NULL, or has a value of 0, the ordinal column is disabled.

Note: This option is only available to Azure SQL Database, Azure SQL Managed Instance, Azure Synapse Analytics (serverless SQL pool only), and SQL Server 2022 (16.x) and later versions.

Return Types

If the ordinal output column isn't enabled, STRING_SPLIT returns a single-column table whose rows are the substrings. The name of the column is value. It returns nvarchar if any of the input arguments are either nvarchar or nchar. Otherwise, it returns varchar. The length of the return type is the same as the length of the string argument.

If the enable_ordinal argument is passed a value of 1, a second column named ordinal is returned that consists of the 1-based index values of each substring's position in the input string. The return type is bigint.

To use STRING_SPLIT to separate a string using a space as the separator, the Transact-SQL statement will look as follows:

DECLARE @s VARCHAR(100)

SET @s = 'the sky is blue';

SELECT value
FROM STRING_SPLIT(@s, ' ', 1);
value
the
sky
is
blue

The second and third step, namely to reverse the order of the list and merge the reversed list of strings separating each word with a space, can be accomplished with the STRING_AGG string function.

The STRING_AGG string function concatenates the values of string expressions and places separator values between them, which is a space in this case. The separator isn't added at the end of string.

Syntax

STRING_AGG ( expression, separator ) [ <order_clause> ]

<order_clause> ::=   
    WITHIN GROUP ( ORDER BY <order_by_expression_list> [ ASC | DESC ] )

Arguments

expression

Is an expression of any type. Expressions are converted to NVARCHAR or VARCHAR types during concatenation. Non-string types are converted to NVARCHAR type.

separator

Is an expression of NVARCHAR or VARCHAR type that is used as separator for concatenated strings. It can be literal or variable.

<order_clause>

Optionally specify order of concatenated results using WITHIN GROUP clause:

WITHIN GROUP ( ORDER BY <order_by_expression_list> [ ASC | DESC ] )

<order_by_expression_list>

A list of non-constant expressions that can be used for sorting results. Only one order_by_expression is allowed per query. The default sort order is ascending.

Using the STRING_AGG string function to merge a list of strings in sorted in reverse order separating each word with a space, and adding it to the query earlier that uses the STRING_SPLIT to split a string of words separated by space, here's how the final query will look like that will reverse words in a string:

DECLARE @s VARCHAR(100)

SET @s = 'the sky is blue';

SELECT STRING_AGG(value, ' ') WITHIN GROUP (ORDER BY ordinal DESC) AS [Output]
FROM STRING_SPLIT(@s, ' ', 1)
WHERE value != '';
Output
blue is sky the
  • STRING_SPLIT Sample Uses of the STRING_SPLIT String Function