Strings & Hashing (Easy)Practice common string manipulation patterns and hashing techniques.

Reverse a String

Problem Statement

Given a string, return its reversed version.

Solution

def reverse_string(s):
    return s[::-1]

Check Palindrome String

Problem Statement

Determine if a string reads the same forward and backward.

Solution

def is_palindrome(s):
    return s == s[::-1]

Count Vowels and Consonants

Problem Statement

Count the number of vowels and consonants in a given string.

Solution

`def count_vowels_consonants(s):
    vowels = "aeiouAEIOU"
    v_count = c_count = 0
    for char in s:
        if char.isalpha():
            if char in vowels:
                v_count += 1
            else:
                c_count += 1
    return {"vowels": v_count, "consonants": c_count

Find String Length without len()

Problem Statement

Calculate the length of a string without using the built-in len() function.

Solution

def find_length(s):
    count = 0
    for _ in s:
        count += 1
    return count

Count Word Occurrences

Problem Statement

Count how many times each word appears in a sentence.

Solution

`def count_words(sentence):
    words = sentence.split()
    counts = {

Check Anagram

Problem Statement

Check if two strings are anagrams of each other (contain same characters with same frequency).

Solution

def is_anagram(s1, s2):
    return sorted(s1) == sorted(s2)

Convert to Lowercase

Problem Statement

Convert all characters in a string to lowercase without using .lower().

Solution

def to_lowercase(s):
    result = ""
    for char in s:
        if 'A' <= char <= 'Z':
            result += chr(ord(char) + 32)
        else:
            result += char
    return result

Convert to Uppercase

Problem Statement

Convert all characters in a string to uppercase without using .upper().

Solution

def to_uppercase(s):
    result = ""
    for char in s:
        if 'a' <= char <= 'z':
            result += chr(ord(char) - 32)
        else:
            result += char
    return result

Remove Spaces

Problem Statement

Remove all spaces from a given string.

Solution

def remove_spaces(s):
    return "".join(s.split())

Replace Character

Problem Statement

Replace all occurrences of a character in a string with another character.

Solution

def replace_char(s, old, new):
    result = ""
    for char in s:
        if char == old:
            result += new
        else:
            result += char
    return result

Find First Non-Repeating Character

Problem Statement

Find the first character in a string that does not repeat.

Solution

`def first_unique(s):
    counts = {

Count Characters Frequency

Problem Statement

Count the frequency of each character in a string.

Solution

`def char_frequency(s):
    freq = {

Check Substring

Problem Statement

Determine if a string contains a specific substring.

Solution

def check_substring(s, sub):
    return sub in s

Find Longest Word

Problem Statement

Find the longest word in a given sentence.

Solution

def find_longest_word(sentence):
    words = sentence.split()
    if not words: return ""
    longest = words[0]
    for word in words:
        if len(word) > len(longest):
            longest = word
    return longest

Remove Duplicate Characters

Problem Statement

Remove duplicate characters from a string while maintaining order.

Solution

def remove_duplicates(s):
    seen = set()
    result = []
    for char in s:
        if char not in seen:
            seen.add(char)
            result.append(char)
    return "".join(result)

Reverse Words in String

Problem Statement

Reverse the order of words in a given sentence.

Solution

def reverse_words(sentence):
    return " ".join(sentence.split()[::-1])

Check Valid Parentheses (Basic)

Problem Statement

Check if a string of parentheses (one type only) is balanced.

Solution

def is_balanced(s):
    count = 0
    for char in s:
        if char == '(':
            count += 1
        elif char == ')':
            count -= 1
        if count < 0:
            return False
    return count == 0

Find ASCII Value

Problem Statement

Find the ASCII value of a character.

Solution

def get_ascii(char):
    return ord(char)

Sort Characters

Problem Statement

Sort the characters of a string alphabetically.

Solution

def sort_string(s):
    return "".join(sorted(s))

Count Digits in String

Problem Statement

Count the total number of numeric digits in a string.

Solution

def count_digits(s):
    count = 0
    for char in s:
        if char.isdigit():
            count += 1
    return count

Remove Vowels

Problem Statement

Remove all vowels from a string.

Solution

def remove_vowels(s):
    vowels = "aeiouAEIOU"
    return "".join([char for char in s if char not in vowels])

Check Rotation of String

Problem Statement

Check if one string is a rotation of another.

Solution

def is_rotated_string(s1, s2):
    if len(s1) != len(s2): return False
    return s2 in (s1 + s1)

Find Common Characters

Problem Statement

Find characters that appear in two strings (intersection).

Solution

def common_characters(s1, s2):
    return "".join(set(s1) & set(s2))

Count Special Characters

Problem Statement

Count symbols and special characters in a string (non-alphanumeric).

Solution

def count_special_chars(s):
    count = 0
    for char in s:
        if not char.isalnum() and not char.isspace():
            count += 1
    return count

Capitalize First Letter

Problem Statement

Capitalize the first letter of each word in a string.

Solution

def capitalize_words(s):
    return s.title()

Swap Two Strings

Problem Statement

Swap the contents of two strings (conceptual or using variables).

Solution

def swap_strings(s1, s2):
    return s2, s1

Find Substring Index

Problem Statement

Find the starting index of a substring or -1 if not found.

Solution

def find_substring(s, sub):
    return s.find(sub)

Check Pangram

Problem Statement

Check if a string contains every letter of the alphabet at least once.

Solution

def is_pangram(s):
    letters = set()
    for char in s.lower():
        if 'a' <= char <= 'z':
            letters.add(char)
    return len(letters) == 26

Remove Punctuation

Problem Statement

Remove all punctuation from a string.

Solution

import string
def remove_punctuation(s):
    return "".join([char for char in s if char not in string.punctuation])

Find Repeated Words

Problem Statement

Find all words that appear more than once in a sentence.

Solution

`def repeated_words(sentence):
    words = sentence.split()
    counts = {

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Strings & Hashing (Easy) / Reverse a String

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Reverse a String

Given a string, return its reversed version.

1
Use slicing with a step of -1 to reverse the string quickly.
2
Or, convert to a list, swap elements from both ends, and join back.
3
Iterating backwards and building a new string is also an option.

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