I am trying to analyze the Time Complexity of a recursive algorithm that solves the Generate all sequences of bits within Hamming distance t problem. The algorithm is this:
// str is the bitstring, i the current length, and changesLeft the
// desired Hamming distance (see linked question for more)
void magic(char* str, int i, int changesLeft) {
if (changesLeft == 0) {
// assume that this is constant
printf("%s\n", str);
return;
}
if (i < 0) return;
// flip current bit
str[i] = str[i] == '0' ? '1' : '0';
magic(str, i-1, changesLeft-1);
// or don't flip it (flip it again to undo)
str[i] = str[i] == '0' ? '1' : '0';
magic(str, i-1, changesLeft);
}
What is the time complexity of this algorithm?
I fond myself pretty rusty when it comes to this and here is my attempt, which I feel is no where near the truth:
t(0) = 1
t(n) = 2t(n - 1) + c
t(n) = t(n - 1) + c
= t(n - 2) + c + c
= ...
= (n - 1) * c + 1
~= O(n)
where n
is the length of the bit string.
It's exponential :
t(0) = 1
t(n) = 2 t(n - 1) + c
t(n) = 2 (2 t(n - 2) + c) + c = 4 t (n - 2) + 3 c
= 2 (2 (2 t(n - 3) + c) + c) + c = 8 t (n - 3) + 7 c
= ...
= 2^i t(n-i) + (2^i - 1) c [at any step i]
= ...
= 2^n t(0) + (2^n - 1) c = 2^n + (2^n - 1) c
~= O(2^n)
Or, using WolframAlpha: https://www.wolframalpha.com/input/?i=t(0)%3D1,+t(n)%3D2+t(n-1)+%2B+c
The reason it's exponential is that your recursive calls are reducing the problem size by 1, but you're making two recursive calls. Your recursive calls are forming a binary tree.
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