Second Largest Digit in a String - Practice Coding Problems

Second Largest Digit in a String - Problem

Given an alphanumeric string s, return the second largest numerical digit that appears in s, or -1 if it does not exist.

An alphanumeric string is a string consisting of lowercase English letters and digits.

Input & Output

Example 1 — Basic Mixed String

$ Input: s = "dfa12321afd"

› Output: 2

💡 Note: Digits present: 1, 2, 3. Largest is 3, second largest is 2.

Example 2 — Duplicate Digits

$ Input: s = "abc1111"

› Output: -1

💡 Note: Only digit 1 appears, so there's no second largest digit.

Example 3 — No Digits

$ Input: s = "abcdef"

› Output: -1

💡 Note: No digits in the string, return -1.

Constraints

1 ≤ s.length ≤ 500
s consists of lowercase English letters and digits.

Visualization

Tap to expand

Understanding the Visualization

Input

Alphanumeric string with digits and letters mixed

Extract

Identify all digit characters (0-9)

Find

Return second largest unique digit or -1

Key Takeaway

🎯 Key Insight: Only 10 possible digits exist, so we can efficiently track which ones appear and find the second largest.

Asked in

a Amazon 25 G Google 18

The key insight is that we only need to track which digits 0-9 appear in the string. The optimal approach uses either a boolean array to mark digit presence or two variables to track the largest and second largest in a single pass. Time: O(n), Space: O(1).

Common Approaches

Approach	Time	Space	Notes
✓ Digit Frequency Tracking	O(n)	O(1)	Use array to track which digits appear, find second largest
Single Pass Two Variables	O(n)	O(1)	Track largest and second largest in one pass through string
Sort All Digits	O(n log n)	O(n)	Extract all digits, sort them, and return second largest

Digit Frequency Tracking — Algorithm Steps

Create boolean array for digits 0-9
Mark digits that appear in string
Find second largest by iterating backwards

Visualization

Tap to expand

Step-by-Step Walkthrough

Mark

Mark which digits are present

Scan

Scan from 9 to 0

Find

Return second digit found

Code -

solution.c — C

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

int solution(char *s) {
    // Track which digits appear
    bool digitPresent[10] = {false};
    
    for (int i = 0; s[i]; i++) {
        if (isdigit(s[i])) {
            digitPresent[s[i] - '0'] = true;
        }
    }
    
    // Find largest and second largest
    int largest = -1;
    int secondLargest = -1;
    
    for (int digit = 9; digit >= 0; digit--) {
        if (digitPresent[digit]) {
            if (largest == -1) {
                largest = digit;
            } else {
                secondLargest = digit;
                break;
            }
        }
    }
    
    return secondLargest;
}

int main() {
    char s[1000];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    int result = solution(s);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through string O(n) plus constant time to check 10 digits

✓ Linear Growth

Space Complexity

O(1)

Fixed size boolean array of 10 elements for digits 0-9

✓ Linear Space

28.4K Views

Medium Frequency

~10 min Avg. Time

892 Likes

Ln 1, Col 1

Smart Actions

💡 Explanation

AI Ready

💡 Suggestion Tab to accept Esc to dismiss

// Output will appear here after running code

Code Editor Closed

Click the red button to reopen

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Digit Frequency Tracking — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Select Compiler