大神帮小弟我看看这个小的查找，哪里出错了

大神帮我看看这个小的查找，哪里出错了？
public class Search{
public static void main(String [] args) {
int a[] = {1, 3, 4, 7, 9, 20, 58, 79 };
int i = 9;
System.out.println(twofen(a,i));
} 
}
public static int twofen(int []a,int num) {
if(a.length == 0)return -1;
int firstpos = 0;
int lastpos = a.length -1;
int m = (firstpos + lastpos) / 2;
while(firstpos <= lastpos){
if(num == a[m])return m;
if(num < a[m]){lastpos = a[m]-1;}
if(num > a[m]){firstpos = a[m]+1;}
m = (firstpos + lastpos) / 2;
}
return -1;
}

[解决办法]
你明显那个部分if没有返回值
while(firstpos <= lastpos){
if(num == a[m])return m;
if(num < a[m]){lastpos = (m-1)/2;}
if(num > a[m]){firstpos = (m+1)/2;}
m = (firstpos + lastpos) / 2;
return -1;
}
修改成
while(firstpos <= lastpos){
if(num == a[m])return m;
if(num < a[m]){lastpos = (m-1)/2;}
if(num > a[m]){firstpos = (m+1)/2;}
m = (firstpos + lastpos) / 2;
return m;}
}
return -1;
}

[解决办法]
刚才那个有点小错误，重发个：

Java code

public class Search{public static void main(String [] args) {int a[] = {1, 3, 4, 7, 9, 20, 58, 79 };int i = -1;System.out.println(twofen(a,i));}  public static int twofen(int []a,int num) {if(a.length == 0)return -1;int firstpos = 0;int lastpos = a.length -1;while(firstpos <=lastpos){if(num == a[(firstpos + lastpos) / 2])return (firstpos + lastpos) / 2;if(num < a[(firstpos + lastpos) / 2]){lastpos = ((firstpos + lastpos) / 2)-1;}if(num > a[(firstpos + lastpos) / 2]){firstpos = ((firstpos + lastpos) / 2)+1;}}return -1;}}
[解决办法]
将if(num  < a[m]){ 
    lastpos = a[m]-1; 
 } 
 if(num >  a[m]){ 
    firstpos = a[m]+1; 
 }两名改为： 
 if(num  < a[m]){ 
    lastpos = m-1; 
 } 
 if(num >  a[m]){ 
    firstpos = m+1; 
 } 
  

[解决办法]
还有你好像多了一个大括号
[解决办法]
1，“int twofen(int []a,int num)”应将此函数应放在class Search里，而不是外面
2，“if(num < a[m]){lastpos = a[m]-1;}”须将 a[m]-1改为m-1
   及“if(num > a[m]){firstpos = a[m]+1;}“里的a[m]+1改为m+1即可
[解决办法]
Java code
public class Search{    public static void main(String [] args) {        int a[] = {1, 3, 4, 7, 9, 20, 58, 79 };        int i = 9;        System.out.println(twofen(a,i));    }      public static int twofen(int []a,int num) {        if(a.length == 0) return -1;        int firstpos = 0;        int lastpos = a.length -1;        int m = (firstpos + lastpos) / 2;        while(firstpos <= lastpos){            if(num == a[m]) return m;            if(num < a[m]){    lastpos = m-1;}            if(num > a[m]){    firstpos = m+1;}            m = (firstpos + lastpos) / 2;        }        return -1;    }}