After sorting (bubble sorting), learn to continuously update other sorting methods

= = = = = = = = = Bubble sort = = = = = = = =

One 、 Bubble sorting means that a maximum or minimum number can be selected for each round of comparison （ Make them bubble ）

Two 、 After each round of comparison, put the largest number after that round of comparison at the end
example ：int[] arr = { 9 , 5 , 3 , 18, 2 } （ Take small to large platoon as an example ）
1、 In the first round, all elements are compared in pairs in order , If the value of the previous element Larger than the value of the following element , Then switch places , After such a round, the last value is the largest
The first round of comparison ：
= = = = = = =9 , 5 , 3 , 18, 2 = = = = = = = =
for the first time 9 and 5 Compare ,9 Big , In exchange for ：
5 , 9 , 3 , 18, 2
The second time 9 and 3 Compare ,9 Big , In exchange for ：
5 , 3 , 9 , 18, 2
third time 9 and 18 Compare ,18 Big , Don't swap ：
5 , 3 , 9 , 18, 2
The fourth time 18 and 2 Compare ,18 Big , In exchange for ：
5 , 3 , 9 , 2,18
The first round of comparison is over , Compared four times , Get a maximum number .

2、 The second round repeats the process of the first round , Only the largest number is placed at the end , There is no need to compare the last number .
The second round of comparison ：
= = = = = = = = = =5 , 3 , 9 , 2,18 = = = = = = = = =
for the first time 5 and 3 Compare ,5 Big , In exchange for ：
3, 5 , 9 , 2, 18
The second time 5 and 9 Compare ,9 Big , Don't swap ：
3 , 5 , 9 , 2 , 18
third time 9 and 2 Compare ,9 Big , In exchange for ：
5 , 3 , 2 , 9, 18

The second round of comparison is over , Compared three times , Get the second largest number .

3、 The third round repeats the process of the second round , Only the largest number and the second largest number are placed at the end , There is no need to compare the last number .

The third round of comparison ：
= = = = = = = = = =5 , 3 , 2 ,9, 18 = = = = = = = = =
for the first time 5 and 3 Compare ,5 Big , In exchange for ：
3, 5 , 2 , 9, 18
The second time 5 and 2 Compare ,5 Big , In exchange for ：
3 , 2 , 5 , 9, 18
The third round of comparison is over , Compared twice , Get a third largest number .

4、 The fourth round repeats the process of the third round , Only the largest number 、 The second largest number and the third largest number are placed at the end , There is no need to compare the last number .
Fourth round comparison ：
= = = = = = = = = = 3 , 2 , 5 , 9, 18 = = = = = = = = =
for the first time 3 and 2 Compare ,3 Big , In exchange for ：
**2 **, 3 , 5 , 9, 18

thus , All the numbers have been compared .

3、 ... and 、 Come to the rule ：
1、 If there is n Number , that We need to compare n-1 round
2、 In comparison k round when , Just compare == n - k == Time .
such as ： Compare the second round , Need to compare 5 - 2 = 3 Time . Because a maximum number has been compared in the first round , Left at this time 4 Number , Just compare 3 Time .

Four 、 Code implementation
Core code ：

int [ ] arr = { 9,5,3,18,2};

	for (int i = 0 ; i < arr.length-1  ;i++ ) {// Control the number of cycles   , 5  Number  ,4  round 
  				
			for (int j = 0 ; j < arr.length - 1 - i  ;j++ ) {  After each round of comparison, the last number will not be compared 
			if(arr[j] >= arr[j+1]){  // If the number in front is larger than that in the back, it will be exchanged , All the way to the end 
				   temp = arr[j];// Exchange value 
 				 arr[j] = arr[j+1];
 				arr[j+1] = temp;
 			}
			
 		}
 }