C++二分查找别写错了！

　　在C++中，二分查找是一个很好用的玩意，主打一个快（平均时间复杂度log n），前提是序列要有序。But，我估计很多人都写错过，或者正在写错的路上（像我）……

 　　 多的不说，直接上错误案例：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l <= r){
 8         mid = (l+r)/2;
 9         if(a[mid] < target)l = mid+1;
10         else if(a[mid] > target)r = mid-1;
11     }
12     cout << mid << " " << a[mid];
13     return 0;
14 }

第二个：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l < r){ //改了这 
 8         mid = (l+r)/2;
 9         if(a[mid] < target)l = mid+1;
10         else if(a[mid] > target)r = mid-1;
11     }
12     cout << mid << " " << a[mid];
13     return 0;
14 }

 

 第三个：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l < r){ 
 8         mid = (l+r)/2;
 9         if(a[mid] < target)l = mid;//改了这 
10         else if(a[mid] > target)r = mid;//改了这 
11     }
12     cout << mid << " " << a[mid];
13     return 0;
14 }

卡死循环了

 
第四个：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l <= r){ //改了这 
 8         mid = (l+r)/2;
 9         if(a[mid] < target)l = mid;//改了这 
10         else if(a[mid] > target)r = mid;//改了这 
11     }
12     cout << mid << " " << a[mid];
13     return 0;
14 }

卡死循环了

第五个：

#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
    int l = 1,r = 20;
    int target,mid;target = 6; //目标值为6 
    while(l <= r){
        mid = (l+r+1)/2;//改了这 
        if(a[mid] < target)l = mid;//改了这 
        else if(a[mid] > target)r = mid;//改了这 
    }
    cout << mid << " " << a[mid];
    return 0;
}

卡死循环了

第六个：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l < r){//改了这 
 8         mid = (l+r+1)/2;//改了这 
 9         if(a[mid] < target)l = mid;//改了这 
10         else if(a[mid] > target)r = mid;//改了这 
11     }
12     cout << mid << " " << a[mid];
13     return 0;
14 }

卡死循环了

打住打住，这六个案例已经能说明问题了：

只要输出在外面，怎么改都是错（包括把>改成>=,<改成<=）

所以，输出一定要放在循环里面！

比如这样：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l <= r){
 8         mid = (l+r+1)/2;
 9         if(a[mid] < target)l = mid+1;
10         else if(a[mid] > target)r = mid-1;
11         else{
12             cout << mid << " " << a[mid];
13             break;
14         }
15     }
16     
17     return 0;
18 }

 

或这样：

#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
    int l = 1,r = 20;
    int target,mid;target = 6; //目标值为6 
    while(l <= r){
        mid = (l+r)/2;//改了这
        if(a[mid] < target)l = mid+1;
        else if(a[mid] > target)r = mid-1;
        else{
            cout << mid << " " << a[mid];
            break;
        }
    }
    
    return 0;
}


但是，这样不行：

#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
    int l = 1,r = 20;
    int target,mid;target = 6; //目标值为6 
    while(l < r){//改了这
        mid = (l+r)/2;//和这
        if(a[mid] < target)l = mid+1;
        else if(a[mid] > target)r = mid-1;
        else{
            cout << mid << " " << a[mid];
            break;
        }
    }
    
    return 0;
}

 问题出在l == r，但是又没找到，所以没有输出

这样也不行：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int a[25] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
 5     int l = 1,r = 20;
 6     int target,mid;target = 6; //目标值为6 
 7     while(l <= r){
 8         mid = (l+r)/2;
 9         if(a[mid] <= target)l = mid+1;//改了这 
10         else if(a[mid] >= target)r = mid-1;//改了这 
11         else{
12             cout << mid << " " << a[mid];
13             break;
14         }
15     }
16     
17     return 0;
18 }

前面两个if和else if把情况全部囊括了，一点都不留给else，这能对吗？

除了这两个，其他的应该都是对的了，吧？（请各位帮我验证一下，结果写在评论区）