Problem Description

You are given a 0-indexed array of strings called details. Each string in this array contains passenger information compressed into exactly 15 characters with a specific format:

• Characters 0-9 (first 10 characters): Phone number
• Character 10 (11th character): Gender
• Characters 11-12 (12th and 13th characters): Age (two-digit number)
• Characters 13-14 (last 2 characters): Seat number

Your task is to count how many passengers are strictly more than 60 years old.

For example, if a string is "9876543210M65A1", the passenger's phone number is "9876543210", gender is "M", age is 65, and seat is "A1". Since 65 > 60, this passenger would be counted.

The solution extracts characters at positions 11 and 12 (using x[11:13]) to get the age portion, converts it to an integer, and checks if it's greater than 60. The sum() function counts all passengers meeting this criteria by treating True as 1 and False as 0.

Intuition

Since each passenger's information follows a fixed format where the age always appears at the same position (characters 11-12), we don't need to parse the entire string or use complex string manipulation. We can directly access the age portion using string slicing.

The key insight is that Python's string slicing x[11:13] gives us exactly the two characters representing the age. Converting this substring to an integer allows us to perform the numerical comparison > 60.

Instead of using a traditional loop with a counter variable, we can leverage Python's generator expression combined with sum(). When we write sum(int(x[11:13]) > 60 for x in details), each comparison int(x[11:13]) > 60 produces a boolean value (True or False). Python treats True as 1 and False as 0 when summing, effectively counting how many passengers meet our age criterion.

This approach is both concise and efficient - we make a single pass through the array, extract only the necessary information (the age), and count in one expression without maintaining explicit state or counter variables.

Solution Approach

The solution uses a simple traversal and counting approach. We iterate through each string x in the details array and extract the age information from the fixed positions.

For each passenger string x:

1. Extract the substring from index 11 to 13 (exclusive) using x[11:13], which gives us the two-character age representation
2. Convert this substring to an integer using int()
3. Check if the age is greater than 60

The implementation uses a generator expression inside the sum() function:

sum(int(x[11:13]) > 60 for x in details)

This works because:

• The generator expression (int(x[11:13]) > 60 for x in details) produces a sequence of boolean values
• Each boolean is True if the passenger's age > 60, otherwise False
• The sum() function treats True as 1 and False as 0
• Therefore, sum() effectively counts the number of True values, giving us the total count of seniors

The time complexity is O(n) where n is the number of passengers, as we visit each string once. The space complexity is O(1) since we only use a constant amount of extra space for the counting operation.

Example Walkthrough

Let's walk through a small example with details = ["7868190130M7522", "5303914400F9211", "9273338290F4010"].

We need to count passengers older than 60 years.

Step 1: Process first passenger "7868190130M7522"

• Extract age using slice [11:13]: "75"
• Convert to integer: 75
• Check if 75 > 60: True
• Running count: 1

Step 2: Process second passenger "5303914400F9211"

• Extract age using slice [11:13]: "92"
• Convert to integer: 92
• Check if 92 > 60: True
• Running count: 2

Step 3: Process third passenger "9273338290F4010"

• Extract age using slice [11:13]: "40"
• Convert to integer: 40
• Check if 40 > 60: False
• Running count: 2

The generator expression (int(x[11:13]) > 60 for x in details) produces the sequence: True, True, False.

When passed to sum(), this becomes: sum([True, True, False]) = sum([1, 1, 0]) = 2.

Therefore, the answer is 2 passengers are strictly older than 60.

Solution Implementation

Time and Space Complexity

The time complexity is O(n), where n is the length of the details list. This is because the code iterates through each element in the list exactly once using a generator expression within the sum() function. For each element, it performs a constant-time operation of string slicing (x[11:13]) and integer comparison (> 60).

The space complexity is O(1). The generator expression used in sum() doesn't create an intermediate list but instead yields values one at a time, consuming constant extra space. The only additional space used is for the counter variable maintained by the sum() function, which remains constant regardless of the input size.

Learn more about how to find time and space complexity quickly.

Common Pitfalls

1. Incorrect String Slicing Indices

One of the most common mistakes is using the wrong indices when extracting the age substring. Since Python uses 0-based indexing, the 12th and 13th characters are at indices 11 and 12, requiring the slice [11:13] (not [12:14] or [11:12]).

Wrong Implementation:

# Incorrect: Only gets one character
age = int(detail[11:12])  # Gets only "6" from "65"

# Incorrect: Wrong position
age = int(detail[12:14])  # Gets "5A" which causes ValueError

Correct Implementation:

age = int(detail[11:13])  # Correctly gets "65"

2. Not Handling Edge Cases in Input Validation

The code assumes all input strings are valid and exactly 15 characters long. In production code, you might encounter malformed data.

Defensive Implementation:

def countSeniors(self, details: List[str]) -> int:
    count = 0
    for detail in details:
        # Validate string length
        if len(detail) != 15:
            continue
      
        # Validate age characters are digits
        age_str = detail[11:13]
        if not age_str.isdigit():
            continue
          
        if int(age_str) > 60:
            count += 1
  
    return count

3. Using >= Instead of > for Age Comparison

The problem specifically asks for passengers "strictly more than 60 years old", meaning age > 60, not age >= 60. A 60-year-old passenger should NOT be counted.

Wrong:

sum(int(detail[11:13]) >= 60 for detail in details)  # Includes 60-year-olds

Correct:

sum(int(detail[11:13]) > 60 for detail in details)  # Excludes 60-year-olds

4. Inefficient String Concatenation or Conversion

Some might try to extract characters individually and concatenate them, which is less efficient.

Inefficient Approach:

age = int(detail[11] + detail[12])  # String concatenation is unnecessary

Better Approach:

age = int(detail[11:13])  # Direct slicing is cleaner and faster

5. Memory-Inefficient Alternative Solutions

Creating intermediate lists when not necessary wastes memory.

Memory-Inefficient:

ages = [int(detail[11:13]) for detail in details]  # Creates full list
return sum(1 for age in ages if age > 60)

Memory-Efficient (Original Solution):

return sum(int(detail[11:13]) > 60 for detail in details)  # Generator expression