Binary Search---First Position of Target

给定一个排序的整数数组（升序）和一个要查找的整数target，用O(logn)的时间查找到target第一次出现的下标（从0开始），如果target不存在于数组中，返回-1。

在数组 [1, 2, 3, 3, 4, 5, 10] 中二分查找3，返回2。

class Solution {
/**
* @param nums: The integer array.
* @param target: Target to find.
* @return: The first position of target. Position starts from 0.
*/
public int binarySearch(int[] nums, int target) {
//write your code here
if (nums==null||nums.length==0) {
return -1;
}
int start = 0, end = nums.length-1;
while(start+1<end){//防止无间隔 导致死循环
int mid = start + (end - start)/2;
if (nums[mid] ==target) {
end =mid;
}else if (nums[mid]<target) {
start = mid;
}else {
end = mid;
}
}
if (nums[start] == target) {
return start;
}
if (nums[end] == target) {
return end;
}
return -1;

}
}