Some answers I wrote here, not necessarily highest scored or ordered:

Algorithmic solutions

• Dimensionality reduction using PCA

• Self Avoiding Walk solution

• Recursion optimization using memoization

• Expected size of the largest connected component in a random simple graph (N nodes, N edges)

Computer vision & image processing

• Classifying corn kernels as infected or not

• Find and apply affine transformation

• Applying homography with and without opencv

• Salt&Pepper denoising + inpainting