Number of Senior Citizens

Number of Senior Citizens - Problem

Array String Easy

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.

Input & Output

Example 1 — Mixed Ages

$ Input: details = ["7868190130M7522","5303914400F9211","9273338640F4317"]

› Output: 2

💡 Note: First passenger: age 75 > 60 (senior), second passenger: age 92 > 60 (senior), third passenger: age 43 ≤ 60 (not senior). Total seniors: 2

Example 2 — No Seniors

$ Input: details = ["1313579440F2036","2921522980M5644"]

› Output: 0

💡 Note: First passenger: age 20 ≤ 60, second passenger: age 56 ≤ 60. No seniors found.

Example 3 — All Seniors

$ Input: details = ["9751302862F0693","3888560693F7262"]

› Output: 1

💡 Note: First passenger: age 06 ≤ 60, second passenger: age 72 > 60. Only one senior found.

Constraints

1 ≤ details.length ≤ 100
details[i].length == 15
details[i] consists of digits and an uppercase English letter
All the phone numbers and seat numbers in the given strings are distinct

Visualization

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Asked in

a Amazon 15 M Microsoft 8

The key insight is that passenger age is always stored at positions 11-12 in the 15-character string. Best approach is direct string parsing with substring extraction. Time: O(n), Space: O(1)

Common Approaches

✓ Direct String Parsing

⏱️ Time: O(n) Space: O(1)

Iterate through each passenger detail string, extract the age from positions 11-12, convert to integer, and count how many are greater than 60.

Direct String Parsing — Algorithm Steps

Step 1: Iterate through each passenger detail string
Step 2: Extract age substring from positions 11-12
Step 3: Convert age to integer and check if > 60
Step 4: Increment counter for senior citizens

Visualization

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Step-by-Step Walkthrough

Parse String

Extract age from positions 11-12

Check Age

Compare if age > 60

Count

Increment counter for seniors

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int solution(char details[][16], int n) {
    int count = 0;
    for (int i = 0; i < n; i++) {
        char ageStr[3];
        ageStr[0] = details[i][11];
        ageStr[1] = details[i][12];
        ageStr[2] = '\0';
        int age = atoi(ageStr);
        if (age > 60) {
            count++;
        }
    }
    return count;
}

int main() {
    char line[1000];
    fgets(line, sizeof(line), stdin);
    
    char details[100][16];
    int n = 0;
    
    // Simple JSON parsing for array of strings
    char *ptr = line;
    while (*ptr && *ptr != '[') ptr++;
    ptr++; // skip [
    
    while (*ptr && *ptr != ']') {
        if (*ptr == '"') {
            ptr++; // skip opening quote
            int j = 0;
            while (*ptr && *ptr != '"' && j < 15) {
                details[n][j++] = *ptr++;
            }
            details[n][j] = '\0';
            n++;
            if (*ptr == '"') ptr++; // skip closing quote
        }
        ptr++;
    }
    
    int result = solution(details, n);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single loop through n passengers, constant time operations per passenger

✓ Linear Growth

Space Complexity

O(1)

Only using a counter variable, no extra data structures

✓ Linear Space

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Input & Output

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Related Problems

Common Approaches

Direct String Parsing — Algorithm Steps

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Code -

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