← Google Interview Insights

Google·Machine Learning Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Interviewed for an ML engineer role at Google and got hit with a conceptual question about CNNs vs RNNs. Pretty standard for this kind of role but still made me second-guess how deep to go.

Questions Asked (1)

Q1

What are the key differences between Convolutional Neural Networks and Recurrent Neural Networks?

Technical Trade-offsAlgorithms & Data Structures
Author's notes

I went with the spatial vs sequential angle, CNNs are built for grid-like data where local patterns matter, RNNs are for sequences where order and context carry meaning.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by defining CNNs and RNNs in one sentence each, then contrast them across key dimensions like data type, architecture, parameter sharing, and use cases. Emphasize that the choice depends on the problem structure: spatial vs. sequential data.

Pro tip: Mention that CNNs can be used for sequential data (e.g., WaveNet) and RNNs for spatial data (e.g., image captioning), showing you understand that the boundaries are not rigid and that hybrid models exist.

1. Define each network type

Briefly define CNN as designed for grid-like data (e.g., images) using convolution and pooling, and RNN as designed for sequential data (e.g., time series) using recurrent connections with memory.

2. Compare data assumptions

Highlight that CNNs assume spatial locality and translation invariance, while RNNs assume temporal dependencies and variable-length sequences.

3. Contrast architectural features

Discuss differences in parameter sharing (CNNs share weights across spatial locations, RNNs share across time steps), depth (CNNs are feedforward, RNNs have loops), and memory (RNNs have hidden state, CNNs do not).

4. Discuss training and challenges

Mention that CNNs are easier to parallelize and less prone to vanishing gradients, while RNNs suffer from vanishing/exploding gradients and are harder to parallelize due to sequential dependencies.

5. Summarize use cases and trade-offs

Conclude with typical applications: CNNs for image classification, object detection; RNNs for language modeling, speech recognition. Note that transformers are increasingly replacing RNNs for sequences.

Key Points to Mention

  • CNNs use convolution and pooling for spatial feature extraction; RNNs use recurrent connections for temporal modeling.
  • Parameter sharing: CNNs share weights across spatial locations; RNNs share weights across time steps.
  • CNNs process fixed-size inputs (e.g., images); RNNs handle variable-length sequences.
  • RNNs maintain a hidden state (memory) across time steps; CNNs do not have inherent memory.
  • Training: CNNs are parallelizable; RNNs are sequential and suffer from vanishing/exploding gradients.
  • Use cases: CNNs for computer vision; RNNs for NLP and time series, though transformers are now dominant.

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