Most Frequent Number Following Key In an Array

Most Frequent Number Following Key In an Array

What is this problem about?

The Most Frequent Number Following Key In an Array problem gives you an integer array and a key value. Find the integer that most frequently appears immediately after occurrences of the key. This Most Frequent Number Following Key In an Array coding problem is a clean frequency counting problem with a simple neighbor-tracking approach.

Why is this asked in interviews?

Google asks this easy-level problem to test attention to detail in array traversal — specifically looking at the element immediately after each occurrence of the key. It validates basic hash map usage and clear problem understanding. The array, hash table, and counting interview pattern is applied directly.

Algorithmic pattern used

Single pass with frequency map. Traverse the array. For each index i where arr[i] == key and i+1 < n, increment freq[arr[i+1]]. After the full pass, return the key of freq with the maximum value.

Example explanation

Array: [1, 100, 200, 1, 100], key = 1.

• Index 0: arr[0]=1=key → arr[1]=100. freq={100:1}.
• Index 3: arr[3]=1=key → arr[4]=100. freq={100:2}. Most frequent follower of key 1 = 100.

Array: [2, 2, 2, 2, 3], key = 2.

• Followers: 2, 2, 2, 3 → freq={2:3, 3:1}. Answer = 2.

Common mistakes candidates make

• Checking arr[i+1] == key instead of arr[i] == key.
• Not handling the edge case where key appears at the last index (no follower).
• Returning the frequency count instead of the actual number.
• Not initializing the frequency map before the loop.

Interview preparation tip

Neighbor-tracking problems follow a simple template: scan the array, check a condition on the current element, act on the next element. Here: "if current == key, count the next element." Write this out explicitly before coding to avoid the common mix-up of checking the wrong index. Easy problems in interviews are opportunities to demonstrate clean, bug-free code written with confidence — take them seriously.