How does iterative deepening affect time complexity?

I have a tree traversal algorithm which generally operates in O(bm) where b is the branching factor and m is the max depth.

Using iterative deepening, this algorithm is run over and over again, m times at increasing depth:

O(bm) = b⁰ + b¹ + b² + ... + bm

Based on my limited understanding of time complexity, we take the largest element because that is the most significant one over time, and so that element would be bm, where m is the max depth reached.

So, with that knowledge I would conclude that the iterative deepening algorithm also runs in O(bm).

However I have trouble understanding, from a logical standpoint, how the tree traversal could have the exact same time complexity whether the algorithm is run once at depth m, or m times up until depth m.

bm is inherently less than Σk=0,..,mbk. Therefore shouldn't the time complexity on iterative deepening be higher?

Or is there something I don't understand?

Answers


Basically you're asking why the following two functions have the same time complexity in terms of big O (as they're both O(n^m)):

  • n^0 + n^1 + n^2 + ... + n^m
  • n^m

The reason is that, at some point, for values of n and m the term n^m dwarfs all the other terms of these functions. As the input grows the runtime of the function as a whole will be determined by n^m.

Even if you come up with a function that takes n^m + 1000000000000 then at some point n^m will still be the deciding term in how long it takes to run. In other words, the runtime of both functions is bounded by a function with the term n^m times some constant.

In your example, running tree traversal once at depth m or running it m times up until depth m have the same time complexity because, as the tree grows, the runtime of running at depth m dwarfs all the other runs. Looking at how long it takes to run at depth m is basically all that is needed to find a function that bounds the runtime of both tasks.

To give another example, we can think of two functions that are both O(n):

  • f1(n) = 1000000000n + 5
  • f2(n) = 3n

It may seem that f1 does more work as n grows so it's not "fair" to say both are O(n). But their time complexity is bounded by a linear function: for some (large) constant c I can say that the runtime of both functions will be below f(n) = cn and thus the time complexity as a whole is O(n).


"Exact same" time complexity is not the same as "taking the exact time". Saying "Exact same time complexity" is like saying "growing with the same speed, up to a constant factor", which is a rough estimation.

For example, if b = 3, you are comparing these two sequences of numbers:

3^m,             or (1, 3,  9, 27,  81, ...)
3^0+3^1+...+3^m, or (1, 4, 13, 40, 121, ...)

Let's denote the first number D(m) (for "DFS"), and the second one I(m) (for "iterative deepening"), then

I(m) = 3/2 * D(m) - 1/2

It is certainly true that I(m) is greater than D(m), but they have the same "Order" of growth. This idea is denoted by I(m) = O(D(m)).

Mathematically, I(m) = O(D(m)), because there exists such a constant k that I(m) < k * D(m) for all m; here k = 3/2.


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