int start = 0; end = nums.length -1;
//相邻就退出
//start = 1, end = 2 就退出
//int mid = (start + end) /2
//start, end ~ 2^31
//Consider special case: [2,2], target = 2
//mid = (0 + 1) / 2 = 0;
// start < end
//dead loop
while (start + 1 < end) {
int mid = (end - start) /2 + start;
if (target == nums[mid]) {
end = mid;
} else if (target < nums[mid]) {
} else { start = mid}
//first position, 先看 start, if last position, 先看end
if (nums[start] = target) {
return start;
}
if (nums[end] == target) {
return end;
}
//start ..............mid..............end
//               target