Is Number of Senior Citizens easy or hard?

Number of Senior Citizens is classified as an Easy problem on LeetCode with a high acceptance rate above 80%. The main challenge is recognizing the fixed string format and correctly extracting the age field.

Number of Senior Citizens Python/Java solution

Both Python and Java implementations follow the same logic: read characters at index 11 and 12, compute the age, and count values greater than 60. Python often uses slicing like s[11:13], while Java typically uses char access and arithmetic to compute the age.

How to solve Number of Senior Citizens in O(n)?

Iterate through the array of passenger detail strings and read the age stored at positions 11 and 12. Convert those characters into an integer and check if the value is greater than 60. Increment a counter for each qualifying record, resulting in a single-pass O(n) solution.

What is the best approach for Number of Senior Citizens?

The character-based calculation approach is typically the best. It directly reads the two age digits from indices 11 and 12 and computes the number without creating a substring. This keeps the solution O(n) time and O(1) space while avoiding extra parsing work.

Is Number of Senior Citizens asked at Google/Amazon/Meta?

Problems of this style appear frequently in screening rounds at large tech companies. While this exact question is categorized as easy, similar fixed-format string parsing tasks are common in interviews at companies like Amazon and Google.

What data structure is used in Number of Senior Citizens?

The problem mainly uses arrays and strings. The input is an array of encoded passenger strings, and the solution involves iterating through the array and extracting characters from each string.

What is the time complexity of Number of Senior Citizens?

The time complexity is O(n), where n is the number of passenger records. Each record is scanned once to read the age digits and compare them with 60. Space complexity remains O(1) because only a counter and a few temporary variables are used.

Description Hints 2 Solution Video 5:07

2678. Number of Senior Citizens

Easy81.5% acceptance2 companies

You are given a 0-indexed array of strings details. Each element of details provides information about a given passenger compressed into a string of length 15. The system is such that:

The first ten characters consist of the phone number of passengers.
The next character denotes the gender of the person.
The following two characters are used to indicate the age of the person.
The last two characters determine the seat allotted to that person.

Return the number of passengers who are strictly more than 60 years old.

Example 1:

Input: details = ["7868190130M7522","5303914400F9211","9273338290F4010"]
Output: 2
Explanation: The passengers at indices 0, 1, and 2 have ages 75, 92, and 40. Thus, there are 2 people who are over 60 years old.

Example 2:

Input: details = ["1313579440F2036","2921522980M5644"]
Output: 0
Explanation: None of the passengers are older than 60.

Constraints:

1 <= details.length <= 100
details[i].length == 15
details[i] consists of digits from '0' to '9'.
details[i][10] is either 'M' or 'F' or 'O'.
The phone numbers and seat numbers of the passengers are distinct.

Asked at 2 companies

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Topics

Array String

Hints open one at a time

Hint 1

Convert the value at index 11 and 12 to a numerical value.

Hint 2

The age of the person at index i is equal to details[i][11]*10+details[i][12].

5:07

Still stuck?Read 3 approaches, or watch a walkthrough.

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LanguageLoading editor…

1class Solution:
2    def countSeniors(self, details: List[str]) -> int:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

["7868190130M7522","5303914400F9211","9273338290F4010"]

Input

["1313579440F2036","2921522980M5644"]

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