二分查找O(logn)

//二分查找O(logn)，要求arr为有序数组
public static int 二分查找(int[] arr,int target){
int start = 0;
int end = arr.length-1;
while(start<=end){
int mid = (start+end)/2;
if(arr[mid] > target){
end = mid-1;
}else if(arr[mid] < target){
start = mid+1;
}else{
return mid;
}
}
return -1;
}

//递归实现

public static int 二分查找(int[] arr,int target,int start,int end){if(start <= end){int mid = (start+end)/2;if(arr[mid] > target){end = mid-1;}else if(arr[mid] < target){start = mid+1;}else{return mid;}return 二分查找(arr,target,start,end);}return -1;}