Hard data structure and algorithm practice puzzle.
Given two words, return the fewest single-letter inserts, deletes, or swaps that turn one into the other. So kitten to sitting is 3. Build a table where cell (i, j) is the cost to turn the first i letters of a into the first j letters of b. If the letters match, copy the diagonal. If not, it's 1 plus the cheapest of the three neighbors. Spell checkers and diff tools use this.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Trip Streak Finder, a pipeline you can run and download. See every Software Engineer project.
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