First Unique Character in a String

The problem is about finding the first non-repeating character in a given string s. If no such character exists, the function should return -1.

A simple approach is to use a dictionary to count the frequency of each character in the string, then iterate through the string again to find the first character that occurs only once.

Here’s the code:

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class Solution(object):
    def firstUniqChar(self, s: str) -> int:
        char_count = {}

        # Count the frequency of each character in the string
        for char in s:
            char_count[char] = char_count.get(char, 0) + 1

        # Find the index of the first character that occurs only once
        for i, char in enumerate(s):
            if char_count[char] == 1:
                return i

        # If no such character exists, return -1
        return -1

This code first iterates through the string to count the frequency of each character, storing the counts in the char_count dictionary. Then it iterates through the string again, checking the count for each character, and returns the index of the first character with a count of 1. If no such character is found, it returns -1.

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class Solution(object):
    def firstUniqChar(self, s: str) -> int:
        hset = collections.Counter(s);

        for idx in range(len(s)):
            if hset[s[idx]] == 1:
                return idx

        return -1

Problem Classification

Here are some ways to classify this problem:

String Manipulation: This problem requires traversing and manipulating a string to find the first non-repeating character. Understanding how strings are indexed and how to work with individual characters is critical.
Data Structure Usage: A data structure (like a dictionary or an array) might be beneficial for tracking character counts and finding the first unique character.
Order Maintenance: This problem requires not just identifying unique characters, but identifying the first one that appears in the string, which means the order of processing is important.
Search Problem: At its core, this problem is about searching through a string to find a specific condition (the first non-repeating character). Understanding various searching techniques may be beneficial.

Language Agnostic Coding Drills

Here’s how the problem can be broken down into its smallest units of learning and thinking step by step from the problem statement to the final solution.

Understanding Data Structures: The problem requires some basic knowledge of data structures like lists (arrays) and dictionaries (hash maps). The key-value nature of dictionaries is exploited in this problem.
Understanding the Counter class in Python: This is a Python specific data structure which is a subclass of dictionary. It is used for counting hashable objects.
Looping through a string: Understanding how to iterate through a string in Python using range and length methods.
Accessing elements in a string: Understand how to access elements of a string using their index.
Accessing and Understanding Dictionary values: This step involves understanding how to access the values stored in a dictionary using keys. Here, each character in the string is a key, and the number of times it appears is the corresponding value.
Condition checking inside a loop: The implementation of a condition inside a loop to check if a character appears only once.
Return statements inside and outside loops: This involves understanding the control flow in Python with return statements, i.e., how they can immediately terminate a function and output a value.
Problem-specific concept: Finally, there is the problem-specific concept of finding and returning the index of the first non-repeating character in a string. If no such character is found, -1 is returned.

The implementation can be broken down into these conceptual parts, each of which can be practiced and mastered individually. After understanding these parts, they can be combined to form the final solution to the problem.

Targeted Drills in Python

Understanding Data Structures:

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# Drill: Initialize an empty list and a dictionary and print them.
list_example = []
dict_example = {}
print(list_example)
print(dict_example)

Understanding the Counter class in Python:

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# Drill: Import the Counter class from the collections module and use it to count the characters in a string.
from collections import Counter
string = "hello world"
counter = Counter(string)
print(counter)

Looping through a string:

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# Drill: Use a for loop to iterate over a string and print each character.
for char in "hello world":
    print(char)

Accessing elements in a string:

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# Drill: Access and print the first and last character of a string.
string = "hello world"
print(string[0])  # first character
print(string[-1])  # last character

Accessing and Understanding Dictionary values:

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# Drill: Initialize a dictionary with some key-value pairs and then print out the value of a given key.
dictionary = {"apple": 1, "banana": 2, "cherry": 3}
print(dictionary["banana"])  # prints: 2

Condition checking inside a loop:

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# Drill: Write a loop that prints only the even numbers in a list.
numbers = [1, 2, 3, 4, 5, 6]
for number in numbers:
    if number % 2 == 0:
        print(number)

Return statements inside and outside loops:

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# Drill: Write a function that returns the first even number in a list, or -1 if there are no even numbers.
def first_even(numbers):
    for number in numbers:
        if number % 2 == 0:
            return number
    return -1

Problem-specific concept:

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# Drill: Use the Counter class to count the characters in a string, then return the first character that has a count of 1.
from collections import Counter
def first_unique_char(string):
    counter = Counter(string)
    for char in string:
        if counter[char] == 1:
            return char
    return -1

Each drill here represents a key concept necessary to implement the final solution. By mastering each of these drills, you’ll be able to integrate them all into a coherent solution to the problem at hand.