问题
First, here's my Shell sort code (using Java):
public char[] shellSort(char[] chars) {
int n = chars.length;
int increment = n / 2;
while(increment > 0) {
int last = increment;
while(last < n) {
int current = last - increment;
while(current >= 0) {
if(chars[current] > chars[current + increment]) {
//swap
char tmp = chars[current];
chars[current] = chars[current + increment];
chars[current + increment] = tmp;
current -= increment;
}
else { break; }
}
last++;
}
increment /= 2;
}
return chars;
}
Is this a correct implementation of Shell sort (forgetting for now about the most efficient gap sequence - e.g., 1,3,7,21...)? I ask because I've heard that the best-case time complexity for Shell Sort is O(n). (See http://en.wikipedia.org/wiki/Sorting_algorithm). I can't see this level of efficiency being realized by my code. If I added heuristics to it, then yeah, but as it stands, no.
That being said, my main question now - I'm having difficulty calculating the Big O time complexity for my Shell sort implementation. I identified that the outer-most loop as O(log n), the middle loop as O(n), and the inner-most loop also as O(n), but I realize the inner two loops would not actually be O(n) - they would be much less than this - what should they be? Because obviously this algorithm runs much more efficiently than O((log n) n^2).
Any guidance is much appreciated as I'm very lost! :P
回答1:
The worst-case of your implementation is Θ(n^2) and the best-case is O(nlogn) which is reasonable for shell-sort.
The best case ∊ O(nlogn):
The best-case is when the array is already sorted. The would mean that the inner if statement will never be true, making the inner while loop a constant time operation. Using the bounds you've used for the other loops gives O(nlogn). The best case of O(n) is reached by using a constant number of increments.
The worst case ∊ O(n^2):
Given your upper bound for each loop you get O((log n)n^2) for the worst-case. But add another variable for the gap size g. The number of compare/exchanges needed in the inner while is now <= n/g. The number of compare/exchanges of the middle while is <= n^2/g. Add the upper-bound of the number of compare/exchanges for each gap together: n^2 + n^2/2 + n^2/4 + ... <= 2n^2 ∊ O(n^2). This matches the known worst-case complexity for the gaps you've used.
The worst case ∊ Ω(n^2):
Consider the array where all the even positioned elements are greater than the median. The odd and even elements are not compared until we reach the last increment of 1. The number of compare/exchanges needed for the last iteration is Ω(n^2).
回答2:
Insertion Sort
If we analyse
static void sort(int[] ary) {
int i, j, insertVal;
int aryLen = ary.length;
for (i = 1; i < aryLen; i++) {
insertVal = ary[i];
j = i;
/*
* while loop exits as soon as it finds left hand side element less than insertVal
*/
while (j >= 1 && ary[j - 1] > insertVal) {
ary[j] = ary[j - 1];
j--;
}
ary[j] = insertVal;
}
}
Hence in case of average case the while loop will exit in middle
i.e 1/2 + 2/2 + 3/2 + 4/2 + .... + (n-1)/2 = Theta((n^2)/2) = Theta(n^2)
You saw here we achieved (n^2)/2 even though divide by two doesn't make more difference.
Shell Sort is nothing but insertion sort by using gap like n/2, n/4, n/8, ...., 2, 1 mean it takes advantage of Best case complexity of insertion sort (i.e while loop exit) starts happening very quickly as soon as we find small element to the left of insert element, hence it adds up to the total execution time.
n/2 + n/4 + n/8 + n/16 + .... + n/n = n(1/2 + 1/4 + 1/8 + 1/16 + ... + 1/n) = nlogn (Harmonic Series)
Hence its time complexity is some thing close to n(logn)^2
来源：https://stackoverflow.com/questions/12767588/time-complexity-for-shell-sort