排序算法 （sorting algorithm）之 冒泡排序（bubble sort）
http://www.algolist.net/Algorithms/
https://docs.oracle.com/javase/tutorial/collections/algorithms/
https://en.wikipedia.org/wiki/Sorting_algorithm
冒泡排序（Bubble sort）
https://en.wikipedia.org/wiki/Bubble_sort
loop1:
4,6,1,3,7 -> 4,6,1,3,7
4,6,1,3,7 -> 4,1,6,3,7
4,1,6,3,7 -> 4,1,3,6,7
4,1,3,6,7 -> 4,1,3,6,7
loop2:
4,1,3,6,7 -> 1,4,3,6,7
1,4,3,6,7 -> 1,3,4,6,7
1,3,4,6,7 -> 1,3,4,6,7
1,3,4,6,7 -> 1,3,4,6,7
loop3:
1,3,4,6,7 -> 1,3,4,6,7
1,3,4,6,7 -> 1,3,4,6,7
1,3,4,6,7 -> 1,3,4,6,7
1,3,4,6,7 -> 1,3,4,6,7
当第三次循环，没有发生swap 说明已排序完成 ，应 break
冒泡排序特点：
1）比较相邻的两个数
2）只能通过判断没有交换来提前结束
最好的情况：
loop1:
1,2,3 -> 1,2,3
1,2,3 -> 1,2,3
最坏的情况：
loop1:
3,2,1 -> 2,3,1
2,3,1 -> 2,1,3
loop2:
2,1,3 -> 1,2,3
1,2,3 -> 1,2,3
loop3:
1,2,3 -> 1,2,3
1,2,3 -> 1,2,3
package sorting; import java.util.Arrays; import org.junit.Test; public class BubbleSorting { int[] items = { 4, 6, 1, 3, 7 }; int step = 0; // ① 相邻 // ② 差一步 // ③ n个数可产生 n-1 对 @Test public void sort() { for (;;) { boolean swapped = false; for (int i = 0; i < items.length - 1; i++) { step++; if (items[i] > items[i + 1]) { swap(i, i + 1); swapped = true; } } if (!swapped) break; } System.out.println(step + ":" + Arrays.toString(items)); } public void swap(int i, int j) { int backup = items[i]; items[i] = items[j]; items[j] = backup; } }
优化1（砍掉最后一个）
Now, the array is already sorted, but the algorithm does not know if it is completed. The algorithm needs one whole pass without any swap to know it is sorted.
loop1:
4,6,1,3,7 -> 4,6,1,3,7
4,6,1,3,7 -> 4,1,6,3,7
4,1,6,3,7 -> 4,1,3,6,7
4,1,3,6,7 -> 4,1,3,6,7
loop2:
4,1,3,6 -> 1,4,3,6
1,4,3,6 -> 1,3,4,6
1,3,4,6 -> 1,3,4,6
loop3:
1,3,4 -> 1,3,4
1,3,4 -> 1,3,4
无swap 结束
package sorting; import java.util.Arrays; import org.junit.Test; public class BubbleSorting { int[] items = { 4, 6, 1, 3, 7 }; int step = 0; // ① 相邻 // ② 差一步 // ③ n个数可产生 n-1 对 // ④ 把最大(小)数移到末尾，n = n -1 来缩小循环次数 @Test public void sort() { int l = items.length; for (;;) { boolean swapped = false; for (int i = 1; i < l; i++) { step++; if (items[i - 1] > items[i]) { swap(i - 1, i); swapped = true; } } l = l - 1; if (!swapped) break; } System.out.println(step + ":" + Arrays.toString(items)); } public void swap(int i, int j) { int backup = items[i]; items[i] = items[j]; items[j] = backup; } }
优化2（砍掉最后一段）
loop1:
4,6,1,3,7 -> 4,6,1,3,7
4,6,1,3,7 -> 4,1,6,3,7
4,1,6,3,7 -> 4,1,3,6,7
4,1,3,6,7 -> 4,1,3,6,7
loop2:
4,1,3 -> 1,4,3
1,4,3 -> 1,3,4
loop3:
1,3 -> 1,3
package sorting; import java.util.Arrays; import org.junit.Test; public class BubbleSorting { int[] items = { 4,6,1,3,7 }; int step = 0; // ① 相邻 // ② 差一步 // ③ n个数可产生 n-1 对 // ④ 找到最后一次交换下标，只保留前面部分 // ⑤ 当 最后一次交换下标 == 0 时，说明没有交换，则终止循环 @Test public void sort() { int l = items.length; for (;;) { int lastSwapIndex = 0; for (int i = 1; i < l; i++) { step++; if (items[i - 1] > items[i]) { swap(i - 1, i); lastSwapIndex = i; } } l = lastSwapIndex; if (l < 1) break; } System.out.println(step + ":" + Arrays.toString(items)); } public void swap(int i, int j) { int backup = items[i]; items[i] = items[j]; items[j] = backup; } }
时过境迁重写 2023-03-07
public static void main(String[] args) { int[] arr = new int[] { 4, 6, 1, 3, 7 }; bub2(arr, "asc"); for (int i : arr) { System.out.println(i); } } /** * 冒泡排序 * * @param arr * @param flag */ public static void bub1(int[] arr, String flag) { boolean asc = "asc".equalsIgnoreCase(flag); boolean exchanged = false; do { exchanged = false; for (int i = 0; i < arr.length - 1; i++) { int a = arr[i]; int b = arr[i + 1]; if (asc ? a > b : a < b) { arr[i] = b; arr[i + 1] = a; exchanged = true; } } } while (exchanged); } /** * 冒泡排序-尾部未发生排序的部分截除 * * @param arr * @param flag */ public static void bub2(int[] arr, String flag) { boolean asc = "asc".equalsIgnoreCase(flag); boolean exchanged = false; int f = arr.length - 1; do { exchanged = false; int e = f; for (int i = 0; i < e; i++) { int a = arr[i]; int b = arr[i + 1]; if (asc ? a > b : a < b) { arr[i] = b; arr[i + 1] = a; f = i; exchanged = true; } } } while (exchanged); }
