Bubble sort - alternate sort at both ends

• Problem description

  Write a two-way bubble sorting algorithm , Scan alternately in both positive and negative directions , That is, put the element with the largest keyword at the end of the first pass , The second time, put the element with the smallest keyword at the top of the sequence , It goes on and on .

• Algorithmic thought

On the basis of the original bubbling sort, it is reformed

Transformation Division ：① External cycle conditions , Because after a round of sorting, the sequence to be sorted , The start and end points of the next sort will change , So don't fill in the increment or decrement statements directly
for(i=0,j=L.length-1;i<j;)
② Internal circulation conditions
for(k=i+1;k<=j;k++) // The largest element is pushed to the last （j The point position is the end position ）
for(l=j-1;l>=i;--l)// The smallest element is pushed to the front

• Complete test code

#include<stdio.h>
int main(){
    
	int a[7]={
    111,112,6,15,3,118,6};
	int i,j,k,l,temp; 
	for(i=0,j=6;i<j;){
    
		int flag=0;// Judge whether there is reverse order 
		for(k=i+1;k<=j;k++){
    
			if(a[k]<a[k-1]){
    
				flag=1;
				temp=a[k];
				a[k]=a[k-1];
				a[k-1]=temp;
			}	
		}
		if(!flag) break;// Is an ordered sequence exiting the loop 
		j--;
		for(l=j-1;l>=i;--l){
    
			if(a[l]>a[l+1]){
    
				flag=1;
				temp=a[l];
				a[l]=a[l+1];
				a[l+1]=temp;
			}	
		}
		if(!flag) break;
		i++;// The subscript needs to be modified because the smallest element will be sorted from the back to the front 
	}
	for(i=0;i<7;i++) 
		printf(" %d ",a[i]);
}