TOP k问题的nlogn代码框架，所有top k题目都适用，用的就是quick select叫快速选择，是快排的变种。
private int quickSort(int[] nums, int start, int end, int k) {
if (start == end) {
return nums[start];
}
int pivot = nums[(start + end)/2];
int left = start; //左指针
int right = end; //右指针
while(left <= right) {
while(left <= right && nums[left] < pivot) {
left++;
}
while(left <= right && nums[right] > pivot) {
right--;
}
if(left <= right) {
int temp = nums[left];
nums[left] = nums[right];
nums[right] = temp;
left++;
right--;
}
}
if(start + k - 1 <= right) {
return quickSort(nums, start, right, k);
}
if(start + k - 1 >= left) {
return quickSort(nums, left, end, k - left + start);
}
return nums[right + 1];
}
