LeetCode 509 Fibonacci - Memoized Code

Here's the complete memoized Fibonacci:

function fib(n, dp)
    if dp[n] ≠ -1 then
        return dp[n]
    if n ≤ 1 then
        return n
    dp[n] := fib(n - 1, dp) + fib(n - 2, dp)
    return dp[n]

You added just three lines: the lookup check, the storage line, and passing dp through recursive calls. The rest is identical to your original recursive solution. Three extra lines. Before: if dinosaurs had started computing fib($40$), we would still be waiting for the answer. After: finishes in the time it takes to blink. This same pattern appears in game AI, route planning, stock trading. You will use it for years.