Question

I deduced the time complexity of bubble sort in its best case according to the mothod used in book ALGORITHMS 2.2. But the answer turned out to be O(n^2).

Here’s my derivation, hope anyone can help me find out where is wrong:

public void bubbleSort(int arr[]) {
for(int i = 0, len = arr.length; i < len - 1; i++) {
    for(int j = 0; j < len - i - 1; j++) {
        if(arr[j + 1] < arr[j])
            swap(arr, j, j + 1);
    }
}

}

Statements                      cost    times
i = 0,len = arr.length          c1          1
i < len - 1                     c2          n
i++                             c3          n - 1
j = 0                           c4          n - 1
j < len - i - 1                 c5          t1(i=0) + t1(i=1) + ... + t1(i = n-2)
j++                             c6          t2(i=0) + t2(i=1) + ... + t2(i = n-2)
arr[j + 1] < arr[j]             c7          t3(i=0) + t3(i=1) + ... + t3(i = n-2)
swap(arr, j, j + 1)             c8          t4(i=0) + t4(i=1) + ... + t4(i = n-2)

T(n) = c1 + c2n + c3(n – 1) + c4(n – 1) + c5t5 + c6t6 + c7t7 + c8t8
= c1 + c2n + c3(n – 1) + c4(n – 1) + c5[t1(i=0) + t1(i=1) + … + t1(i = n-2)] + c6[t2(i=0) + t2(i=1) + … + t2(i = n-2)] + c7[t3(i=0) + t3(i=1) + … + t3(i = n-2)] + c8[t4(i=0) + t4(i=1) + … + t4(i = n-2)];

in its best cast, the sequence is already positive before sorting. Then t8 sould be 0.

T(n) = c1 + c2n + c3(n – 1) + c4(n – 1) + c5[t1(i=0) + t1(i=1) + … + t1(i = n-2)] + c6[t2(i=0) + t2(i=1) + … + t2(i = n-2)] + c7[t3(i=0) + t3(i=1) + … + t3(i = n-2)]

The time complexity is O(n^2)

Answer 1

Your implementation

public void bubbleSort(int arr[]) {
    for(int i = 0, len = arr.length; i < len - 1; i++) {
        for(int j = 0; j < len - i - 1; j++) {
            if(arr[j + 1] < arr[j])
                swap(arr, j, j + 1);
        }
    }
}

lacks the control whether there was any swap in the inner loop, and the breaking out of the outer loop if there wasn’t.

That control makes it possible that the best case (an already sorted array) is O(n), since then there are no swaps in the inner loop when it runs the first time.

public void bubbleSort(int arr[]) {
    boolean swapped = true;
    for(int i = 0, len = arr.length; swapped && i < len - 1; i++) {
        swapped = false;
        for(int j = 0; j < len - i - 1; j++) {
            if(arr[j + 1] < arr[j]) {
                swap(arr, j, j + 1);
                swapped = true;
            }
        }
    }
}