Find Lucky Integer in an Array

Find Lucky Integer in an Array - Problem

Given an array of integers arr, a lucky integer is an integer that has a frequency in the array equal to its value.

Return the largest lucky integer in the array. If there is no lucky integer return -1.

Input & Output

Example 1 — Basic Lucky Number

$ Input: arr = [2,2,3,4]

› Output: 2

💡 Note: Number 2 appears 2 times, so 2 is lucky. Number 3 appears 1 time (3 ≠ 1), number 4 appears 1 time (4 ≠ 1). Only 2 is lucky.

Example 2 — No Lucky Numbers

$ Input: arr = [1,2,2,3,3,3]

› Output: 3

💡 Note: Number 1 appears 1 time (1 = 1, lucky), number 2 appears 2 times (2 = 2, lucky), number 3 appears 3 times (3 = 3, lucky). The largest lucky number is 3.

Example 3 — Multiple Lucky Numbers

$ Input: arr = [7,7,7,7,7,7,7]

› Output: 7

💡 Note: Number 7 appears 7 times, so 7 is lucky. This is the only number and it's lucky.

Constraints

1 ≤ arr.length ≤ 500
1 ≤ arr[i] ≤ 500

Visualization

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Asked in

a Amazon 25 G Google 20 M Microsoft 15

The key insight is to count the frequency of each number and check if any number equals its frequency count. The optimal approach uses a hash map to count frequencies in O(n) time, then finds the largest number where value equals frequency. Time: O(n), Space: O(n).

Common Approaches

✓ Sort First

⏱️ Time: N/A Space: N/A

Brute Force Counting

⏱️ Time: O(n²) Space: O(1)

For each unique number in the array, count how many times it appears by scanning the entire array. Check if the count equals the number itself.

Single-Pass with Early Maximum

⏱️ Time: O(n) Space: O(n)

While building the frequency map, immediately check if any number becomes lucky and update the maximum. This optimizes the common case where we find lucky numbers early.

Two-Pass Hash Map

⏱️ Time: O(n) Space: O(n)

Use a hash map to count the frequency of each number in one pass. Then iterate through the frequency map to find numbers where the value equals its frequency, keeping track of the maximum.

Algorithm Steps — Algorithm Steps

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

✓ Linear Growth

Space Complexity

✓ Linear Space

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Medium Frequency

~15 min Avg. Time

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Smart Actions

💡 Explanation

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