Bytedance·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Bytedance coding screen, one algorithmic problem about string manipulation. Pretty focused session, nothing behavioral, just the problem and a bit of back and forth on edge cases.

Questions Asked (1)

You're given two strings of equal length containing only 'x' and 'y'. In a single move, you can swap any character from the first string with any character from the second. What's the minimum number of swaps to make both strings identical, and when is it impossible?

Algorithms & Data Structures

Author's notes

Took me a minute to realize the impossible case isn't just about counts being off.

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AI HintsAI Generated

Suggested Approach

First, clarify that the goal is to make both strings identical by swapping characters between them, and that the final strings must be the same. Then, analyze the mismatched positions: count how many positions have (x,y) and how many have (y,x). The minimum swaps is the sum of the ceiling of half of each count, and it's impossible if the total number of mismatches is odd.

Pro tip: Mention that swapping two mismatched pairs of the same type (e.g., two (x,y) pairs) fixes both in one swap, while mixed pairs require two swaps. This shows you understand the optimization beyond just counting.

1. Understand the problem and constraints

Restate the problem: we have two strings of equal length with only 'x' and 'y'. We can swap any character from the first string with any character from the second. We need the minimum swaps to make both strings identical, and determine when it's impossible.

2. Identify mismatched positions

Compare the strings position by position. Count the number of positions where the first string has 'x' and the second has 'y' (type A), and where the first has 'y' and the second has 'x' (type B).

3. Determine impossibility condition

If the total number of mismatches (type A + type B) is odd, it's impossible to make the strings identical because each swap changes the total number of mismatches by an even number (0 or 2).

4. Compute minimum swaps

For each type, pairs of the same type can be fixed with one swap per pair. If there's an odd leftover of each type, they can be fixed together with two swaps. So minimum swaps = floor(A/2) + floor(B/2) + 2*(A%2).

5. Verify with examples and edge cases

Test with small examples (e.g., 'xy' and 'yx', 'xx' and 'yy') to confirm the formula. Also consider cases with no mismatches (0 swaps) and odd total mismatches (impossible).

Key Points to Mention

The total number of mismatched positions must be even for it to be possible.
Two mismatches of the same type (both (x,y) or both (y,x)) can be resolved in one swap.
A mixed pair of mismatches (one (x,y) and one (y,x)) requires two swaps to resolve.
The minimum swaps formula: floor(A/2) + floor(B/2) + 2*(A%2), where A and B are counts of each mismatch type.
Swapping characters within the same string is not allowed; only swaps between the two strings.
The final strings must be identical, meaning each position must have the same character in both strings.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.