Does prices[i + 1..] panic on the last day?

No. A range that starts at prices.len() is a valid empty slice, so the inner loop simply does nothing. Only a start past the length panics. Iterating a slice instead of for j in i + 1..prices.len() also avoids the needless_range_loop lint and per-index bounds checks.

Why not take the max minus the min of the iterator?

prices.iter().max() and .min() ignore which day came first: on vec![4, 11, 2, 8] they give 9, a trade that sells before it buys, while the answer is 7. They also return Option, and unwrap() on an empty vector panics with "called Option::unwrap() on a None value".

Can the subtraction overflow in a debug build?

Not here. Debug builds panic with "attempt to subtract with overflow", but lowest is updated before p - lowest, so the difference is between two real prices and is never negative. With prices up to 10⁴, i32 is plenty.

Is there an iterator version with fold?

Yes: prices.iter().fold((i32::MAX, 0), |(low, best), &p| { let low = low.min(p); (low, best.max(p - low)) }).1. The tuple stays on the stack, so it allocates nothing and compiles to the same loop as the for version.

Why borrow with &prices when the function owns the Vec?

Either works. for p in prices consumes the vector, which is fine because nothing uses it afterwards; for &p in &prices borrows it and copies each i32 out with the &p pattern. Borrowing keeps prices usable later, for example to look up the buy day.

Best Time to Buy and Sell Stock in Rust: Brute Force and One-Pass Solutions

max_profit.rs

Rust 1.98

Not run yet

+ 7 hidden

Input

prices = [8, 3, 6, 1, 4, 9, 7]

Expected

Buy on day 3 at 1, sell on day 5 at 9: 9 - 1 = 8.

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far

1 · Brute force: try every buy and sell day

2 · One pass with the lowest price so far