Minimum Number of Steps to Make Two Strings Anagram

Minimum Number of Steps to Make Two Strings Anagram

1. What is this problem about?

Two strings are anagrams if they contain the same characters with the same frequencies. In this problem, you are given two strings of equal length, s and t. You want to find the minimum number of characters in t that you need to replace with other characters to make t an anagram of s. Note that you are only allowed to change characters, not add or delete them.

2. Why is this asked in interviews?

This "Minimum Number of Steps to Make Two Strings Anagram coding problem" is a staple in interviews at companies like Microsoft, Amazon, and Adobe. It tests fundamental string manipulation and frequency counting techniques. It's an introductory problem that assesses if a candidate can use hash tables or frequency arrays effectively rather than trying to sort the strings, which would be less efficient.

3. Algorithmic pattern used

The algorithmic pattern used is Frequency Counting. By calculating the frequency of each character in both strings, you can determine how many characters in t are "missing" or "extra" relative to s. Since the strings are of equal length, the number of extra characters in t will exactly match the number of replacements needed.

4. Example explanation

• s = "bab", t = "aba"
• Frequency of 'a' in s: 1, in t: 2. (t has 1 extra 'a')
• Frequency of 'b' in s: 2, in t: 1. (t is missing 1 'b') To make t an anagram of s, you must replace the extra 'a' in t with 'b'. Steps = 1.

5. Common mistakes candidates make

Some candidates try to find the Longest Common Subsequence, which is overkill and computationally expensive. Others might try to sort both strings and compare them, which is O(n log n) and doesn't directly give the minimum steps easily. A frequent logic error is summing all differences and forgetting to divide by 2, or simply tracking only the characters that are more frequent in one string.

6. Interview preparation tip

For any "anagram" or "character replacement" problem, the first thing you should think of is a frequency array of size 26. It's almost always the most memory-efficient and fastest way to solve the problem. Practice calculating character counts and comparing them in a single pass.