二叉搜索的各种bugs——重复递增序列

int binary_search(int* A, int value, int p, int r);

int main(int argc, char *argv[]){

        int A[] = {1, 2, 3, 4, 4, 4, 4, 4, 4, 5};

        int index = binary_search(A, 4, 0, 9);

        printf("%d\n", index);

        return 0;
}

int binary_search(int* A, int value, int p, int r){
        if(A==NULL || p>r){
                return -1;
        }

        int q;
        while(p<r){                       
                q = p/2 + r/2;                    
                if(value==A[q]){               
                        if(A[q] == A[q-1]){            
                                r = q-1;                       
                        }
                        else{
                                return q;                      
                        }
                }
                else if(value > A[q]){         
                        p = q+1;                       
                }
                else if(value < A[q]){         
                        r = q-1;                       
                }
        }

        return -1;
}

1 while(p<r) error case:

 

 

查找value 5

q = (p+r)/2 = 4

p = q + 1 = 5

(p < r) != true

while 结束，没有找到5

2 q = p/2 + r/2 error case:

查找value 5

q = p/2 + r/2 = 4

q 超出了p和r的范围，陷入死循环之中

3 q = (p+r)/2 可能会越界

4 A[q] == A[q-1] 可能会越界

 

正确的方案（忽略 p+r 越界）

int binary_search(int* A, int value, int p, int r){
        if(A==NULL || p>r){
                return -1;
        }
                                                                                                                                                     
        int q;                                                                                                                                       
        while(p<=r){                                                                                                                                 
                q = (p + r)/2;                                                                                                                       
                if(value==A[q]){                                                                                                                     
                        if(q>0 && A[q] == A[q-1]){
                                r = q-1;                                                                                                             
                        }                                                                                                                            
                        else{                                                                                                                        
                                return q;                                                                                                            
                        }                                                                                                                            
                }                                                                                                                                    
                else if(value > A[q]){                                                                                                               
                        p = q+1;                                                                                                                     
                }                                                                                                                                    
                else if(value < A[q]){                                                                                                               
                        r = q-1;                                                                                                                     
                }