Second Largest Digit in a String

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

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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

✓ Digit Frequency Tracking

⏱️ Time: O(n) Space: O(1)

Use a boolean array of size 10 to mark which digits (0-9) appear in the string, then iterate backwards to find the largest and second largest digits.

Single Pass Two Variables

⏱️ Time: O(n) Space: O(1)

Maintain two variables for the largest and second largest digits seen so far. Update them as we encounter each digit character in a single pass through the string.

Sort All Digits

⏱️ Time: O(n log n) Space: O(n)

Extract all digit characters from the string, convert to numbers, sort in descending order, and return the second element if it exists and is different from the 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

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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

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Input & Output

Constraints

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Related Problems

Common Approaches

Digit Frequency Tracking — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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