leetcode 215. Kth Largest Element in an Array (medium)

版权声明：by ruohua3kou https://blog.csdn.net/ruohua3kou/article/details/88963223

TopK问题，一般两种解法
1.构建大顶堆
2.利用快排的分治思想
这里采用了第二种方法
关于top-K问题的很详细的解析

class Solution
{
	int partition(vector<int> &nums, int low, int high)
	{
		int l = low + 1, r = high;
		int base = nums[low];
		while (l <= r)
		{
			if (nums[l] < base && nums[r] > base)
			{
				swap(nums[l++], nums[r--]);
			}
			if (nums[l] >= base)
				++l;
			if (nums[r] <= base)
				--r;
		}
		swap(nums[low], nums[r]);
		return r;
	}

  public:
	int findKthLargest(vector<int> &nums, int k)
	{
		int low = 0, high = nums.size() - 1;
		while (true)
		{
			int indexBase = partition(nums, low, high);
			if (indexBase == k - 1)
				return nums[indexBase];
			if (indexBase < k - 1)
				low = indexBase + 1;
			else
				high = indexBase - 1;
		}
		return 0;
	}
};