折半查找
public int bsearch(int[] arr,int left,int right,int v){
int m = 0;
while(left < right){
m =(left+right)/2;
if(arr[m] == v) {
return m;
}
else if(arr[m] > v) {
right = m;
}
else{
left = m+ 1;
}
}
return -1;
}
